diff --git a/.appveyor.yml b/.appveyor.yml
index 3c9ce83..52d0a91 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -6,6 +6,7 @@ environment:
   - julia_version: 1.3
   - julia_version: 1.4
   - julia_version: 1.5
+  - julia_version: 1.6
   - julia_version: nightly
 
 platform:
diff --git a/.travis.yml b/.travis.yml
index f653184..0bac303 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,6 +11,7 @@ julia:
   - 1.3
   - 1.4
   - 1.5
+  - 1.6
   - nightly
 matrix:
   allow_failures:
diff --git a/Project.toml b/Project.toml
index 9c8105d..163bb63 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "UnitSystems"
 uuid = "3a241a3c-2137-41aa-af5e-1388e404ca09"
 authors = ["Michael Reed"]
-version = "0.2.2"
+version = "0.3"
 
 [compat]
 julia = "1"
diff --git a/README.md b/README.md
index ecd07a6..c75c05c 100644
--- a/README.md
+++ b/README.md
@@ -8,10 +8,10 @@
 [![Coverage Status](https://coveralls.io/repos/chakravala/UnitSystems.jl/badge.svg?branch=master&service=github)](https://coveralls.io/github/chakravala/UnitSystems.jl?branch=master)
 [![codecov.io](https://codecov.io/github/chakravala/UnitSystems.jl/coverage.svg?branch=master)](https://codecov.io/github/chakravala/UnitSystems.jl?branch=master)
 
-Specifications for dimensional units are in the [UnitSystems.jl](https://github.com/chakravala/UnitSystems.jl) and [UnitfulSystems.jl](https://github.com/chakravala/UnitfulSystems.jl) repositories.
-The two packages are designed so that they can be interchanged if compatibility with [Unitful.jl](https://github.com/PainterQubits/Unitful.jl) is desired or not.
-However, the `UnitfulSystems` package has fewer `UnitSystem` specifications available than the `UnitSystems` package due to limitations in combination with the `Unitful` package.
-Specifically, `Metric`, `SI2019`, `CODATA`, `Conventional`, `MTS`, `EMU2019`, `English`, and `EnglishUS` can have `Unitful` values; while `Gauss`, `LorentzHeaviside`, `Thomson`, `EMU`, `ESU`, `ESU2019`, `IAU`, `FFF`, `Planck`, `PlanckGauss`, `Stoney`, `Hartree`, `Rydberg`, `Schrodinger`, `Electronic`, `Natural`, `NaturalGauss`, `QCD`, `QCDGauss`, and `QCDoriginal` are plain valued.
+Specifications for dimensional units are in the [UnitSystems.jl](https://github.com/chakravala/UnitSystems.jl) and [MeasureSystems.jl](https://github.com/chakravala/MeasureSystems.jl) and [UnitfulSystems.jl](https://github.com/chakravala/UnitfulSystems.jl) repositories.
+The three packages are designed so that they can be interchanged if compatibility with [Measurements.jl](https://github.com/JuliaPhysics/Measurements.jl) or [Unitful.jl](https://github.com/PainterQubits/Unitful.jl) is desired or not.
+However, the `UnitfulSystems` package has fewer `UnitSystem` specifications available than the `UnitSystems` and `MeasureSystems` packages due to limitations in combination with the `Unitful` package.
+Specifically, `Metric`, `SI2019`, `CODATA`, `Conventional`, `MTS`, `EMU2019`, `English`, and `EnglishUS` can have `Unitful` values; while `Gauss`, `LorentzHeaviside`, `Thomson`, `EMU`, `ESU`, `ESU2019`, `IAU`, `FFF`, `Planck`, `PlanckGauss`, `Stoney`, `Hartree`, `Rydberg`, `Schrodinger`, `Electronic`, `Natural`, `NaturalGauss`, `QCD`, `QCDGauss`, and `QCDoriginal` are plain valued or are limited to `Measurement` values.
 
 ```Julia
 pkg> add UnitSystems # or UnitfulSystems
@@ -55,4 +55,12 @@ Thermodynamics: `temperature`, `entropy`, `specificentropy`, `volumeheatcapacity
 `molarmass`, `molality`, `mole`, `molarity`, `molarvolume`, `molarentropy`, `molarenergy`, `molarconductivity`, `molarsusceptibility`, `catalysis`, `specificity`,
 `luminousflux`, `luminance`, `luminousenergy`, `luminousexposure`, `luminousefficacy`.
 
-Other similar packages include [PhysicalConstants.jl](https://github.com/JuliaPhysics/PhysicalConstants.jl), [MathPhysicalConstants.jl](https://github.com/LaGuer/MathPhysicalConstants.jl), [Unitful.jl](https://github.com/PainterQubits/Unitful.jl.git), [UnitfulSystems.jl](https://github.com/chakravala/UnitfulSystems.jl), [UnitfulUS.jl](https://github.com/PainterQubits/UnitfulUS.jl), [UnitfulAstro.jl](https://github.com/JuliaAstro/UnitfulAstro.jl), [UnitfulAtomic.jl](https://github.com/sostock/UnitfulAtomic.jl), [NaturallyUnitful.jl](https://github.com/MasonProtter/NaturallyUnitful.jl), and [UnitfulMoles.jl](https://github.com/rafaqz/UnitfulMoles.jl).
+**Generalized dimensionless `Coupling`:**
+
+```Julia
+Coupling{αG,α,μₑᵤ,μₚᵤ}
+```
+Specification of `Universe` with the dimensionless `Coupling` constants `coupling`, `finestructure`, `electronunit`, `protonunit`, and `protonelectron`. Alterations to these values can be facilitated and quantified using parametric polymorphism.
+Due to the `Coupling` interoperability, the `MeasureSystems` package is made possible to support calculations with `Measurements` having error standard deviations.
+
+Other similar packages include [MeasureSystems.jl](https://github.com/chakravala/MeasureSystems.jl), [PhysicalConstants.jl](https://github.com/JuliaPhysics/PhysicalConstants.jl), [MathPhysicalConstants.jl](https://github.com/LaGuer/MathPhysicalConstants.jl), [Unitful.jl](https://github.com/PainterQubits/Unitful.jl.git), [UnitfulSystems.jl](https://github.com/chakravala/UnitfulSystems.jl), [UnitfulUS.jl](https://github.com/PainterQubits/UnitfulUS.jl), [UnitfulAstro.jl](https://github.com/JuliaAstro/UnitfulAstro.jl), [UnitfulAtomic.jl](https://github.com/sostock/UnitfulAtomic.jl), [NaturallyUnitful.jl](https://github.com/MasonProtter/NaturallyUnitful.jl), and [UnitfulMoles.jl](https://github.com/rafaqz/UnitfulMoles.jl).
diff --git a/src/UnitSystems.jl b/src/UnitSystems.jl
index c7996aa..1864085 100644
--- a/src/UnitSystems.jl
+++ b/src/UnitSystems.jl
@@ -5,10 +5,6 @@ module UnitSystems
 
 import Base: @pure, length, time
 
-export slug, ft, ftUS, KJ1990, KJ2014, RK1990, RK2014, mₑ1990, mₑ2014, temp, units
-export slugs, kilograms, lbm, meters, feet, rankine, kelvin, moles, molecules
-export UnitSystem, US, SI, MKS, CGS, CGS2019, CGSm, CGSe, HLU, FFF
-
 const Systems = (:Metric,:SI2019,:CODATA,:Conventional,:MTS,:English,:EnglishUS,:IAU,:SI1976,:Mixed,:ESU2019,:EMU2019,:EMU,:ESU,:Gauss,:LorentzHeaviside,:Thomson,:Kennelly,:Planck,:PlanckGauss,:Stoney,:Hartree,:Rydberg,:Schrodinger,:Electronic,:Natural,:NaturalGauss,:QCD,:QCDGauss,:QCDoriginal)
 const Constants = (:hyperfine,:lightspeed,:planck,:planckreduced,:electronmass,:molarmass,:boltzmann,:permeability,:rationalization,:lorentz,:luminousefficacy)
 const Physics = (:atomicmass,:protonmass,:planckmass,:newton,:einstein,:hartree,:rydberg,:bohr,:bohrreduced,:electronradius,:avogadro,:universal,:stefan,:radiationdensity,:permittivity,:coulomb,:ampere,:biotsavart,:charge,:faraday,:impedance,:conductance,:klitzing,:josephson,:magneticflux,:magneton)
@@ -23,6 +19,25 @@ const Convert = [Mechanics...,Electromagnetic...,Thermodynamic...,Molar...,Photo
 
 listext(x) = join(x,"`, `")
 
+@pure measure(x) = x
+
+# universe
+
+"""
+    Coupling{αG,α,μₑᵤ,μₚᵤ}
+
+Specification of `Universe` with the dimensionless `Coupling` constants `coupling`, `finestructure`, `electronunit`, `protonunit`, and `protonelectron`.
+Alterations to these values can be facilitated and quantified using parametric polymorphism.
+Due to the `Coupling` interoperability, the `MeasureSystems` package is made possible to support calculations with `Measurements` having error standard deviations.
+"""
+struct Coupling{αG,α,μₑᵤ,μₚᵤ} end
+@pure coupling(U::Coupling{αG}) where αG = measure(αG)
+@pure finestructure(U::Coupling{αG,α}) where {αG,α} = measure(α)
+@pure electronunit(U::Coupling{αG,α,μₑᵤ}) where {αG,α,μₑᵤ} = measure(μₑᵤ)
+@pure protonunit(U::Coupling{αG,α,μₑᵤ,μₚᵤ}) where {αG,α,μₑᵤ,μₚᵤ} = measure(μₚᵤ)
+@pure protonelectron(U::Coupling) = protonunit(U)/electronunit(U)
+Base.display(U::Coupling) = println("Coupling{αG=$(coupling(U)),α=$(finestructure(U)),μₑᵤ=$(electronunit(U)),μₚᵤ=$(protonunit(U))}")
+
 # unit systems
 
 """
@@ -45,28 +60,36 @@ Thermodynamics: `$(listext(Thermodynamic))`,
 struct UnitSystem{kB,ħ,𝘤,μ₀,mₑ,λ,αL} end
 @pure UnitSystem{k,ħ,𝘤,μ,m,λ}() where {k,ħ,𝘤,μ,m,λ} = UnitSystem{k,ħ,𝘤,μ,m,λ,1}()
 @pure UnitSystem{k,ħ,𝘤,μ,m}() where {k,ħ,𝘤,μ,m} = UnitSystem{k,ħ,𝘤,μ,m,1}()
-@pure boltzmann(::UnitSystem{k}) where k = k
-@pure planckreduced(::UnitSystem{k,ħ}) where {k,ħ} = ħ
-@pure lightspeed(::UnitSystem{k,ħ,𝘤}) where {k,ħ,𝘤} = 𝘤
-@pure permeability(::UnitSystem{k,ħ,𝘤,μ}) where {k,ħ,𝘤,μ} = μ
-@pure electronmass(::UnitSystem{k,ħ,𝘤,μ,m}) where {k,ħ,𝘤,μ,m} = m
-@pure rationalization(::UnitSystem{k,ħ,𝘤,μ,m,λ}) where {k,ħ,𝘤,μ,m,λ} = λ
-@pure lorentz(::UnitSystem{k,ħ,𝘤,μ,m,λ,α}) where {k,ħ,𝘤,μ,m,λ,α} = α
+@pure boltzmann(::UnitSystem{k}) where k = measure(k)
+@pure planckreduced(::UnitSystem{k,ħ}) where {k,ħ} = measure(ħ)
+@pure lightspeed(::UnitSystem{k,ħ,𝘤}) where {k,ħ,𝘤} = measure(𝘤)
+@pure permeability(::UnitSystem{k,ħ,𝘤,μ}) where {k,ħ,𝘤,μ} = measure(μ)
+@pure electronmass(::UnitSystem{k,ħ,𝘤,μ,m}) where {k,ħ,𝘤,μ,m} = measure(m)
+@pure rationalization(::UnitSystem{k,ħ,𝘤,μ,m,λ}) where {k,ħ,𝘤,μ,m,λ} = measure(λ)
+@pure lorentz(::UnitSystem{k,ħ,𝘤,μ,m,λ,α}) where {k,ħ,𝘤,μ,m,λ,α} = measure(α)
 # ΔνCs:s⁻¹, c:m⋅s⁻¹, h:kg⋅m²⋅s⁻¹, kB:kg⋅m²⋅s⁻²⋅K⁻¹, NA:mol⁻¹, Kcd: cd⋅sr⋅s³⋅kg⁻¹⋅m⁻²
 
 isrationalized(U::UnitSystem) = rationalization(U) ≠ 4π
 
 Base.display(U::UnitSystem) = println("UnitSystem{kB=$(boltzmann(U)),ħ=$(planckreduced(U)),𝘤=$(lightspeed(U)),μ₀=$(permeability(U)),mᵤ=$(electronmass(U)),λ=$(isrationalized(U) ? rationalization(U) : "4π"),αL=$(lorentz(U))}")
 
+@pure universe(::UnitSystem) = Universe
 @pure unit(x,y=1) = isapprox(y,x,rtol=eps()^0.9) ? y : x
 @pure mass(U::UnitSystem,S::UnitSystem) = electronmass(U,S)
-@pure electronmass(𝘩::Float64,R∞::Float64=R∞) = αinv^2*R∞*2𝘩/𝘤
-@pure planckmass(U::UnitSystem) = mass(mP,U)
-@pure planck(U::UnitSystem) = 2π*planckreduced(U)
-@pure newton(U::UnitSystem) = lightspeed(U)*planckreduced(U)/planckmass(U)^2
-@pure charge(U::UnitSystem) = sqrt(2planck(U)/impedance(U)/αinv) # fine structure
-@pure impedance(U::UnitSystem) = permeability(U)*lightspeed(U)*rationalization(U)*lorentz(U)^2
-
+@pure electronmass(𝘩::Float64) = αinv^2*R∞*2𝘩/𝘤
+@pure electronmass(𝘩::Float64,C::Coupling) = inv(finestructure(C))^2*R∞*2𝘩/𝘤
+@pure planckmass(U::UnitSystem,C::Coupling=universe(U)) = electronmass(U,C)/√coupling(C)
+@pure planck(U::UnitSystem,C::Coupling=universe(U)) = 2π*planckreduced(U,C)
+@pure newton(U::UnitSystem,C::Coupling=universe(U)) = lightspeed(U,C)*planckreduced(U,C)/planckmass(U,C)^2
+@pure charge(U::UnitSystem,C::Coupling=universe(U)) = sqrt(2planck(U,C)*finestructure(C)/impedance(U,C))
+@pure impedance(U::UnitSystem,C::Coupling=universe(U)) = permeability(U,C)*lightspeed(U,C)*rationalization(U)*lorentz(U)^2
+
+for unit ∈ (:coupling,:finestructure,:electronunit,:protonunit,:protonelectron)
+    @eval @pure $unit(U::UnitSystem) = $unit(universe(U))
+end
+for unit ∈ (:boltzmann,:planckreduced,:lightspeed,:permeability,:electronmass,:molarmass)
+    @eval @pure $unit(U::UnitSystem,C::Coupling) = $unit(U)
+end
 for unit ∈ Constants
     @eval @pure $unit(U::UnitSystem,S::UnitSystem) = unit($unit(S)/$unit(U))
 end
@@ -106,17 +129,18 @@ const kcal = kcalₜₕ; const cal = kcal/1000 # calₜₕ thermal calorie
 
 # fundamental constants, αinv = (34259-1/4366.8123)/250 # 137.036 exactly?
 
-const ΔνCs,Kcd,mP = 9192631770.0,683.002,2.176434e-8 # planck mass (kg)
+const ΔνCs,Kcd,mP = 9192631770.0,683*555.016/555,2.176434e-8 # planck mass (kg)
 const NA,kB,𝘩,𝘤,𝘦 = 6.02214076e23,1.380649e-23,6.62607015e-34,299792458.,1.602176634e-19
 const μₑᵤ,μₚᵤ,αinv,R∞ = 1/1822.888486209,1.007276466621,137.035999084,10973731.5681601
-const αL,μ₀ = 0.01/𝘤,2𝘩/𝘤/αinv/𝘦^2 # ≈ 4π*(1e-7+5.5e-17), exact charge
-const ħ,δμ₀,μₚₑ,Rᵤ,mₑ = 𝘩/2π,μ₀-4π*1e-7,μₚᵤ/μₑᵤ,NA*kB,electronmass(𝘩)
+const mₑ,μ₀ = electronmass(𝘩),2𝘩/𝘤/αinv/𝘦^2 # ≈ 4π*(1e-7+5.5e-17), exact charge
+const ħ,δμ₀,μₚₑ,Rᵤ,αL,αG = 𝘩/2π,μ₀-4π*1e-7,μₚᵤ/μₑᵤ,NA*kB,0.01/𝘤,(mₑ/mP)^2
 const RK1990,RK2014,KJ1990,KJ2014 = 25812.807,25812.8074555,4.835979e14,4.835978525e14
 const ħ1990,ħ2014 = 2/RK1990/KJ1990^2/π,2/RK2014/KJ2014^2/π
 const mₑ1990,mₑ2014 = electronmass(2π*ħ1990),electronmass(2π*ħ2014)
 
 # engineering units # Thomson: αL = 1/2
 
+const Universe = Coupling{αG,1/αinv,μₑᵤ,μₚᵤ}()
 const Gauss = UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,4π,0.01/𝘤}()
 const LorentzHeaviside = UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,1,0.01/𝘤}()
 const Thomson = UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,4π,1/2}()
@@ -137,9 +161,9 @@ const EnglishUS = UnitSystem{1000Rᵤ*mₑ/μₑᵤ*rankine/slug/ftUS^2,ħ/slug/
 
 # astronomical units
 
-const GMsun,GMearth,GMjupiter =  1.32712442099e20,398600441.8e6,1.26686534e17
+const GMsun,GMearth,GMjupiter = 1.32712442099e20,398600441.8e6,1.26686534e17
 const au,LD,day = 149597870.7e3,384402e3,60^2*24
-const pc,ly,GG = au*648000/π,365.25𝘤*day,newton(SI2019)
+const pc,ly,GG = au*648000/π,365.25𝘤*day,𝘤*ħ/mP^2
 const mₛ = GMsun/GG; const Jₛ = mₛ*au^2/day^2; export mₛ,Jₛ,au,day
 const IAU = UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001/Jₛ,ħ/day/Jₛ,day*𝘤/au,4π*1e-7*day^2/Jₛ,mₑ/mₛ}()
 
@@ -152,7 +176,6 @@ const CGS, CGS2019, CGSm, CGSe, HLU = Gauss, EMU2019, EMU, ESU, LorentzHeaviside
 
 # natural units
 
-const αG = (mₑ/mP)^2
 const Planck = UnitSystem{1,1,1,1,√(4π*αG)}()
 const PlanckGauss = UnitSystem{1,1,1,4π,√αG}()
 const Stoney = UnitSystem{1,αinv,1,4π,√(αG*αinv)}()
@@ -168,15 +191,37 @@ const QCDoriginal = UnitSystem{1,1,1,4π/αinv,1/μₚₑ}()
 
 # physical constants
 
+@pure electronmass(U::typeof(Planck),C::Coupling) = sqrt(4π*coupling(C))
+@pure electronmass(U::typeof(PlanckGauss),C::Coupling) = sqrt(coupling(C))
+@pure electronmass(U::UnitSystem{kB,ħ,𝘤,μ₀,√(αG*αinv)},C::Coupling) where {kB,ħ,𝘤,μ₀} = sqrt(coupling(C)/finestructure(C))
+@pure electronmass(U::UnitSystem{kB,ħ,𝘤,μ₀,1/μₚₑ},C::Coupling) where {kB,ħ,𝘤,μ₀} = 1/protonelectron(C)
+@pure permeability(U::UnitSystem{kB,ħ,𝘤,4π/αinv^2},C::Coupling) where {kB,ħ,𝘤} = 4π*finestructure(C)^2
+@pure permeability(U::UnitSystem{kB,ħ,𝘤,π/αinv^2},C::Coupling) where {kB,ħ,𝘤} = π*finestructure(C)^2
+@pure lightspeed(U::UnitSystem{kB,ħ,αinv},C::Coupling) where {kB,ħ} = 1/finestructure(C)
+@pure lightspeed(U::UnitSystem{kB,ħ,2αinv},C::Coupling) where {kB,ħ} = 2/finestructure(C)
+@pure planckreduced(U::UnitSystem{kB,αinv},C::Coupling) where kB = 1/finestructure(C)
+
+@pure electronmass(U::UnitSystem{kB,ħ,𝘤,μ₀,mₑ},C::Coupling) where {kB,ħ,μ₀} = electronmass(planck(U),C)
+@pure electronmass(U::UnitSystem{kB,ħ,100𝘤,μ₀,1000mₑ},C::Coupling) where {kB,ħ,μ₀} = 1000electronmass(SI,C)
+@pure electronmass(U::UnitSystem{kB,ħ,𝘤,μ₀,mₑ/1000},C::Coupling) where {kB,ħ,μ₀} = electronmass(SI,C)/1000
+@pure electronmass(U::UnitSystem{kB,ħ,𝘤,μ₀,mₑ2014},C::Coupling) where {kB,ħ,μ₀} = electronmass(planck(U),C)
+@pure electronmass(U::UnitSystem{kB,ħ,𝘤,μ₀,mₑ1990},C::Coupling) where {kB,ħ,μ₀} = electronmass(planck(U),C)
+@pure electronmass(U::UnitSystem{kB,ħ,𝘤/ftUS,μ₀,mₑ/slug},C::Coupling) where {kB,ħ,μ₀} = electronmass(SI,C)/slug
+@pure permeability(U::UnitSystem{kB,ħ,𝘤,μ₀},C::Coupling) where {kB,ħ,𝘤} = finestructure(C)*2𝘩/𝘤/𝘦^2
+@pure permeability(U::typeof(ESU2019),C::Coupling) = 1e3*permeability(SI,C)/𝘤^2
+@pure permeability(U::typeof(EMU2019),C::Coupling) = 1e7*permeability(SI,C)
+@pure permeability(U::typeof(CODATA),C::Coupling) = 2RK2014*finestructure(C)/𝘤
+@pure permeability(U::typeof(Conventional),C::Coupling) = 2RK1990*finestructure(C)/𝘤
+
 @pure molarmass(U::UnitSystem{1}) = 1
-@pure molarmass(U::UnitSystem{kB}) = electronmass(U)*NA/μₑᵤ
-@pure molarmass(U::UnitSystem{1e7*kB}) = 1000molarmass(SI2019)
-@pure molarmass(U::UnitSystem{1e3*kB}) = molarmass(SI2019)/1000
+@pure molarmass(U::UnitSystem{kB},C::Coupling=universe(U)) = NA*electronmass(U,C)/electronunit(C)
+@pure molarmass(U::UnitSystem{1e7*kB},C::Coupling=universe(U)) = 1000molarmass(SI2019,C)
+@pure molarmass(U::UnitSystem{1e3*kB},C::Coupling=universe(U)) = molarmass(SI2019,C)/1000
 @pure molarmass(U::UnitSystem{kB}) where kB = molarmass(CGS)/1000
 @pure molarmass(U::UnitSystem{boltzmann(MTS)}) = molarmass(CGS)/1e6
 @pure molarmass(U::UnitSystem{boltzmann(CGS)}) = molarmass(Natural)
 @pure molarmass(U::UnitSystem{boltzmann(FFF)}) = molarmass(Natural)
-@pure molarmass(U::UnitSystem{boltzmann(English)}) = 1000molarmass(SI2019)
+@pure molarmass(U::UnitSystem{boltzmann(English)},C::Coupling=universe(U)) = 1000molarmass(SI2019,C)
 @pure molarmass(U::UnitSystem{boltzmann(EnglishUS)}) = molarmass(Natural)
 @pure molarmass(U::UnitSystem{boltzmann(IAU)}) = 1/1000mₛ
 
@@ -187,643 +232,6 @@ include("kinematic.jl")
 include("electromagnetic.jl")
 include("thermodynamic.jl")
 include("physics.jl")
-
-# == Metric is different
-const κ = einstein(SI2019)
-const σ = stefan(SI2019) #
-const μB = magneton(SI2019) #
-const ε₀ = permittivity(SI2019) #
-const kₑ = coulomb(SI2019) #
-const mₚ = protonmass(SI2019)
-const mᵤ = atomicmass(SI2019)
-const Mᵤ = molarmass(SI2019)
-const 𝔉 = faraday(SI2019) #
-const Φ₀ = magneticflux(SI2019) #
-const Z₀ = impedance(SI2019) #
-const G₀ = conductance(SI2019) #
-const Eₕ = hartree(SI2019)
-const a₀ = bohr(SI2019)
-const rₑ = electronradius(SI2019)
-const RK = klitzing(SI2019) #
-const KJ = josephson(SI2019) #
-const RH,Ry = R∞*mₚ/(mₑ+mₚ),𝘩*𝘤*R∞
-
-const ℓP = length(PlanckGauss,SI2019)
-const tP = time(PlanckGauss,SI2019)
-const TP = temperature(PlanckGauss,SI2019)
-
-const lS = length(Stoney,SI2019)
-const tS = time(Stoney,SI2019)
-const mS = mass(Stoney,SI2019)
-const qS = charge(Stoney,SI2019)
-
-const lA = length(Hartree,SI2019)
-const tA = time(Hartree,SI2019)
-const mA = mass(Hartree,SI2019)
-const qA = charge(Hartree,SI2019)
-
-const lQCD = length(QCD,SI2019)
-const tQCD = time(QCD,SI2019)
-const mQCD = mass(QCD,SI2019)
-
-# non standard units
-
-const BTUftlb = 3600/0.5778thermalconductivity(English) # BTU⋅ft⁻¹⋅lb⁻¹
-const BTUJ = energy(English)*BTUftlb # BTU⋅J⁻¹
-
-# constant aliases
-
-const mpe, meu, mpu, ainv, aG = μₚₑ, μₑᵤ, μₚᵤ, αinv, αG
-const Mu,Ru,SB,hh,cc,m0,e0,ke,me,mp,mu,ee,FF,Z0,G0,Eh,a0,re,g0,lP,aL,ϵ₀ = Mᵤ,Rᵤ,σ,𝘩,𝘤,μ₀,ε₀,kₑ,mₑ,mₚ,mᵤ,𝘦,𝔉,Z₀,G₀,Eₕ,a₀,rₑ,g₀,ℓP,αL,ε₀
-export κ, GG, NA, kB, Rᵤ, σ, 𝘩, ħ, 𝘤, μ₀, ε₀, kₑ, mₑ, mₚ, mᵤ, 𝘦, 𝔉, Φ₀, Z₀, G₀, Eₕ, R∞, a₀, rₑ, KJ, RK, Ru, SB, hh, cc, m0, e0, ke, me, mp, mu, ee, FF, Z0, G0, Eh, a0, re, μB
-export αG, αinv, μₚₑ, μₑᵤ, μₚᵤ, mpe, meu, mpu, mP, δμ₀, Mᵤ, Mu, RH, Ry, ΔνCs, Kcd, ainv
-export cal, kcal, calₜₕ, kcalₜₕ, calᵢₜ, kcalᵢₜ, ℓP, g₀, g0, atm, lbm, BTUJ, BTUftlb, aG
-export lP, tP, TP, lS, tS, mS, qS, lA, tA, mA, qA, lQCD, tQCD, mQCD, ϵ₀, αL, aL
-
-# engineering unit systems docs
-
-cgstext(US,AMP,cgs=eval(US)) = """
-```Julia
-julia> boltzmann($US) # erg⋅K⁻¹
-$(boltzmann(cgs))
-
-julia> planckreduced($US) # erg⋅s⋅rad⁻¹
-$(planckreduced(cgs))
-
-julia> lightspeed($US) # cm⋅s⁻¹
-$(lightspeed(cgs))
-
-julia> permeability($US) # statH⋅cm⁻¹
-$(permeability(cgs))
-
-julia> electronmass($US) # g
-$(electronmass(cgs))
-
-julia> rationalization($US)
-$(rationalization(cgs))
-```
-"""
-
-for U ∈ (:CGSm,:CGSe,:EMU,:ESU)
-    (EU,AMP) = QuoteNode.(U ∉ (:CGSe,:ESU) ? (:EMU,:Bi) : (:ESU,:statA))
-@eval @doc """
-    $($(QuoteNode(U)))::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,$($EU≠:EMU ? "(100𝘤)^-2" : 1),1000mₑ,4π}
-
-Centimetre-gram-second `UnitSystem` variant based on `$($EU)` (non-rationalized).
-
-$(cgstext($(QuoteNode(U)),$AMP))
-""" $U
-
-U ∉ (:CGSm,:CGSe) && @eval @doc """
-    $(Symbol($(QuoteNode(U)),:2019))::UnitSystem{1e7*kB,1e7*ħ,100𝘤,$($EU≠:EMU ? "1e3*μ₀/𝘤^2" : "1e7*μ₀"),1000mₑ}
-
-Centimetre-gram-second `UnitSystem` variant of tuned `SI2019` based on `$($EU)` (rationalized).
-
-$(cgstext(Symbol($(QuoteNode(U)),:2019),$AMP))
-""" $(Symbol(U,:2019))
-end
-
-@doc """
-    Thomson::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,4π,1/2}
-
-Centimetre-gram-second `UnitSystem` variant `Thomson` (EMU-Lorentz, non-rationalized).
-
-```Julia
-julia> boltzmann(Thomson) # erg⋅K⁻¹
-$(boltzmann(Thomson))
-
-julia> planckreduced(Thomson) # erg⋅s⋅rad⁻¹
-$(planckreduced(Thomson))
-
-julia> lightspeed(Thomson) # cm⋅s⁻¹
-$(lightspeed(Thomson))
-
-julia> permeability(Thomson) # abH⋅cm⁻¹
-$(permeability(Thomson))
-
-julia> electronmass(Thomson) # g
-$(electronmass(Thomson))
-
-julia> rationalization(Thomson)
-$(rationalization(Thomson))
-
-julia> lorentz(Thomson)
-$(lorentz(Thomson))
-```
-""" Thomson
-
-@doc """
-    Gauss::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,4π,0.01/𝘤}
-
-Centimetre-gram-second `UnitSystem` variant `CGS` (Gauss-Lorentz, non-rationalized).
-
-```Julia
-julia> boltzmann(Gauss) # erg⋅K⁻¹
-$(boltzmann(Gauss))
-
-julia> planckreduced(Gauss) # erg⋅s⋅rad⁻¹
-$(planckreduced(Gauss))
-
-julia> lightspeed(Gauss) # cm⋅s⁻¹
-$(lightspeed(Gauss))
-
-julia> permeability(Gauss) # statH⋅cm⁻¹
-$(permeability(Gauss))
-
-julia> electronmass(Gauss) # g
-$(electronmass(Gauss))
-
-julia> rationalization(Gauss)
-$(rationalization(Gauss))
-
-julia> lorentz(Gauss)
-$(lorentz(Gauss))
-```
-""" Gauss, CGS
-
-@doc """
-    LorentzHeaviside::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,1,0.01/𝘤}
-
-Centimetre-gram-second `UnitSystem` variant `HLU` (Heaviside-Lorentz, rationalized).
-
-```Julia
-julia> boltzmann(LorentzHeaviside) # erg⋅K⁻¹
-$(boltzmann(LorentzHeaviside))
-
-julia> planckreduced(LorentzHeaviside) # erg⋅s⋅rad⁻¹
-$(planckreduced(LorentzHeaviside))
-
-julia> lightspeed(LorentzHeaviside) # cm⋅s⁻¹
-$(lightspeed(LorentzHeaviside))
-
-julia> permeability(HLU) # hlH⋅cm⁻¹
-$(permeability(LorentzHeaviside))
-
-julia> electronmass(LorentzHeaviside) # g
-$(electronmass(LorentzHeaviside))
-
-julia> rationalization(LorentzHeaviside)
-$(rationalization(LorentzHeaviside))
-
-julia> lorentz(LorentzHeaviside)
-$(lorentz(LorentzHeaviside))
-```
-""" LorentzHeaviside, HLU
-
-@doc """
-    MTS::UnitSystem{1e6*Rᵤ*mₑ/μₑᵤ,1000ħ,𝘤,4π/1e4,mₑ/1000}
-
-Metre-tonne-second `UnitSystem` variant of `Metric` system.
-
-```Julia
-julia> boltzmann(MTS) # kJ⋅K⁻¹
-$(boltzmann(MTS))
-
-julia> planckreduced(MTS) # kJ⋅s⋅rad⁻¹
-$(planckreduced(MTS))
-
-julia> lightspeed(MTS) # m⋅s⁻¹
-$(lightspeed(MTS))
-
-julia> permeability(MTS) # kH⋅m⁻¹
-$(permeability(MTS))
-
-julia> electronmass(MTS) # t
-$(electronmass(MTS))
-```
-""" MTS
-
-@doc """
-    Metric::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001,ħ,𝘤,4π*1e-7,mₑ}
-
-Systeme International d'Unites (the SI units) adopted as the preferred `UnitSystem`.
-
-```Julia
-julia> boltzmann(Metric) # J⋅K⁻¹
-$(boltzmann(Metric))
-
-julia> planckreduced(Metric) # J⋅s⋅rad⁻¹
-$(planckreduced(Metric))
-
-julia> lightspeed(Metric) # m⋅s⁻¹
-$(lightspeed(Metric))
-
-julia> permeability(Metric) # H⋅m⁻¹
-$(permeability(Metric))
-
-julia> electronmass(Metric) # kg
-$(electronmass(Metric))
-```
-""" Metric
-
-@doc """
-    SI2019::UnitSystem{kB,ħ,𝘤,μ₀,mₑ}
-
-Systeme International d'Unites (the SI units) with `μ₀` for a tuned `charge` exactly.
-
-```Julia
-julia> boltzmann(SI2019) # J⋅K⁻¹
-$(boltzmann(SI2019))
-
-julia> planckreduced(SI2019) # J⋅s⋅rad⁻¹
-$(planckreduced(SI2019))
-
-julia> lightspeed(SI2019) # m⋅s⁻¹
-$(lightspeed(SI2019))
-
-julia> permeability(SI2019) # H⋅m⁻¹
-$(permeability(SI2019))
-
-julia> electronmass(SI2019) # kg
-$(electronmass(SI2019))
-```
-""" SI2019, SI
-
-@doc """
-    CODATA::UnitSystem{Rᵤ*mₑ2014/μₑᵤ/0.001,2/RK2014/KJ2014^2/π,𝘤,2RK2014/𝘤/αinv,mₑ2014}
-
-Metric `UnitSystem` based on Committee on Data of the International Science Council.
-
-```Julia
-julia> boltzmann(CODATA) # J⋅K⁻¹
-$(boltzmann(CODATA))
-
-julia> planckreduced(CODATA) # J⋅s⋅rad⁻¹
-$(planckreduced(CODATA))
-
-julia> lightspeed(CODATA) # m⋅s⁻¹
-$(lightspeed(CODATA))
-
-julia> permeability(CODATA) # H⋅m⁻¹
-$(permeability(CODATA))
-
-julia> electronmass(CODATA) # kg
-$(electronmass(CODATA))
-```
-""" CODATA
-
-@doc """
-    Conventional::UnitSystem{Rᵤ*mₑ1990/μₑᵤ/0.001,2/RK1990/KJ1990^2/π,𝘤,2RK1990/𝘤/αinv,mₑ1990}
-
-Conventional electronic `UnitSystem` with 1990 tuned `josephson` and `klitzing` constants.
-
-```Julia
-julia> boltzmann(Conventional) # J⋅K⁻¹
-$(boltzmann(Conventional))
-
-julia> planckreduced(Conventional) # J⋅s⋅rad⁻¹
-$(planckreduced(Conventional))
-
-julia> lightspeed(Conventional) # m⋅s⁻¹
-$(lightspeed(Conventional))
-
-julia> permeability(Conventional) # H⋅m⁻¹
-$(permeability(Conventional))
-
-julia> electronmass(Conventional) # kg
-$(electronmass(Conventional))
-```
-""" Conventional
-
-@doc """
-    IAU::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001/Jₛ,ħ*day/Jₛ,day*𝘤/au,4π*1e-7*day^2/Jₛ,mₑ/mₛ}
-
-Astronomical (solar) `UnitSystem` defined by International Astronomical Union.
-
-```Julia
-julia> boltzmann(IAU) # M⊙⋅au²⋅D⁻²⋅K⁻¹
-$(boltzmann(IAU))
-
-julia> planckreduced(IAU) # M⊙⋅au²⋅D⁻¹⋅rad⁻¹
-$(planckreduced(IAU))
-
-julia> lightspeed(IAU) # au⋅D⁻¹
-$(lightspeed(IAU))
-
-julia> permeability(IAU) # M⊙⋅au²⋅C⁻²
-$(permeability(IAU))
-
-julia> electronmass(IAU) # M⊙
-$(electronmass(IAU))
-```
-""" IAU
-
-@doc """
-    English::UnitSystem{kB*rankine/slug/ft^2,ħ/slug/ft^2,𝘤/ft,4π,mₑ/slug}
-
-Engineering `UnitSystem` historically used by Britain and United States.
-
-```Julia
-julia> boltzmann(English) # ft⋅lb⋅°R⁻¹
-$(boltzmann(English))
-
-julia> planckreduced(English) # ft⋅lb⋅s⋅rad⁻¹
-$(planckreduced(English))
-
-julia> lightspeed(English) # ft⋅s⁻¹
-$(lightspeed(English))
-
-julia> permeability(English) # slug⋅ft²⋅?⁻²
-$(permeability(English))
-
-julia> electronmass(English) # slugs
-$(electronmass(English))
-```
-""" English
-
-@doc """
-    EnglishUS::UnitSystem{1000Rᵤ*mₑ/μₑᵤ*rankine/slug/ftUS^2,ħ/slug/ftUS^2,𝘤/ftUS,4π,mₑ/slug}
-
-Engineering `UnitSystem` based on the geophysical US survey foot (1200/3937).
-
-```Julia
-julia> boltzmann(EnglishUS) # ftUS⋅lb⋅°R⁻¹
-$(boltzmann(EnglishUS))
-
-julia> planckreduced(EnglishUS) # ftUS⋅lb⋅s⋅rad⁻¹
-$(planckreduced(EnglishUS))
-
-julia> lightspeed(EnglishUS) # ftUS⋅s⁻¹
-$(lightspeed(EnglishUS))
-
-julia> permeability(EnglishUS) # slug⋅ftUS²⋅?⁻²
-$(permeability(EnglishUS))
-
-julia> electronmass(EnglishUS) # slugs
-$(electronmass(EnglishUS))
-```
-""" EnglishUS
-
-@doc """
-    FFF::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001*rankine/Jf,ħ/14day/Jf,14day*𝘤/201.168,0,mₑ/mf}
-
-Furlong–firkin–fortnight `FFF` is a humorous `UnitSystem` based on unusal impractical units.
-
-```Julia
-julia> boltzmann(FFF) # fir⋅fur²⋅ftn⁻²⋅F⁻¹
-$(boltzmann(FFF))
-
-julia> planckreduced(FFF) # fir⋅fur²⋅ftn⁻¹⋅rad⁻¹
-$(planckreduced(FFF))
-
-julia> lightspeed(FFF) # fur⋅ftn⁻¹
-$(lightspeed(FFF))
-
-julia> permeability(FFF) # fir⋅fur²⋅Inf⁻²
-$(permeability(FFF))
-
-julia> electronmass(FFF) # fir
-$(electronmass(FFF))
-```
-""" FFF
-
-@doc """
-    Kennelly::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001,ħ,𝘤,1e-7,mₑ,4π}
-
-Kennelly ? variant `UnitSystem` of the standard `Metric` units ???
-
-```Julia
-julia> boltzmann(Kennelly) # J⋅K⁻¹
-$(boltzmann(Kennelly))
-
-julia> planckreduced(Kennelly) # J⋅s⋅rad⁻¹
-$(planckreduced(Kennelly))
-
-julia> lightspeed(Kennelly) # m⋅s⁻¹
-$(lightspeed(Kennelly))
-
-julia> permeability(Kennelly) # H⋅m⁻¹
-$(permeability(Kennelly))
-
-julia> electronmass(Kennelly) # kg
-$(electronmass(Kennelly))
-
-julia> rationalization(Kennelly)
-$(rationalization(Kennelly))
-```
-""" Kennelly
-
-# natural unit system docs
-
-textunits(U,S) = """
-```Julia
-julia> boltzmann($S)
-$(boltzmann(U))
-
-julia> planckreduced($S)
-$(planckreduced(U))
-
-julia> lightspeed($S)
-$(lightspeed(U))
-
-julia> permeability($S)
-$(permeability(U))
-
-julia> electronmass($S)
-$(electronmass(U))
-```
-"""
-
-@doc """
-    Planck::UnitSystem{1,1,1,1,√(4π*αG)}
-
-Planck `UnitSystem` with the `electronmass` value `√(4π*αG)` using gravitational coupling.
-
-$(textunits(Planck,:Planck))
-""" Planck
-
-@doc """
-    PlanckGauss::UnitSystem{1,1,1,4π,√αG}
-
-Planck (Gauss) `UnitSystem` with `permeability` of `4π` and `electronmass` coupling `√αG`.
-
-$(textunits(PlanckGauss,:PlanckGauss))
-
-The well known `PlanckGauss` values for `length`, `time`, `mass`, and `temperature` are:
-```Julia
-julia> length(PlanckGauss,Metric) # ℓP
-$(length(PlanckGauss,Metric))
-
-julia> time(PlanckGauss,Metric) # tP
-$(time(PlanckGauss,Metric))
-
-julia> mass(PlanckGauss,Metric) # mP
-$(mass(PlanckGauss,Metric))
-
-julia> temperature(PlanckGauss,Metric) # TP
-$(temperature(PlanckGauss,Metric))
-```
-""" PlanckGauss
-
-@doc """
-    Stoney::UnitSystem{1,αinv,1,4π,√(αG*αinv)}
-
-Stoney `UnitSystem` with `permeability` of `4π` and `electronmass` coupling `√(αG*αinv)`.
-
-$(textunits(Stoney,:Stoney))
-
-The well known `Stoney` values for `length`, `time`, `mass`, and `charge` are:
-```Julia
-julia> length(Stoney,Metric) # lS
-$(length(Stoney,Metric))
-
-julia> time(Stoney,Metric) # tS
-$(time(Stoney,Metric))
-
-julia> mass(Stoney,Metric) # mS
-$(mass(Stoney,Metric))
-
-julia> charge(Stoney,Metric) # qS
-$(charge(Stoney,Metric))
-```
-""" Stoney
-
-@doc """
-    Hartree::UnitSystem{1,1,αinv,4π/αinv^2,1}
-
-Hartree atomic `UnitSystem` with `lightspeed` of `αinv` and `permeability` of `4π/αinv^2`.
-
-$(textunits(Hartree,:Hartree))
-
-The well known `Hartree` atomic unit values for `length`, `time`, `mass`, and `charge` are:
-```Julia
-julia> length(Hartree,Metric) # lA
-$(length(Hartree,Metric))
-
-julia> time(Hartree,Metric) # tA
-$(time(Hartree,Metric))
-
-julia> mass(Hartree,Metric) # mA
-$(mass(Hartree,Metric))
-
-julia> charge(Hartree,Metric) # qA
-$(charge(Hartree,Metric))
-```
-""" Hartree
-
-@doc """
-    Rydberg::UnitSystem{1,1,2αinv,π/αinv^2,1/2}
-
-Rydberg `UnitSystem` with `lightspeed` of `2αinv` and `permeability` of `π/αinv^2`.
-
-$(textunits(Rydberg,:Rydberg))
-""" Rydberg
-
-@doc """
-    Schrodinger::UnitSystem{1,1,αinv,4π/αinv^2,√(αG*αinv)}
-
-Schrodinger `UnitSystem` with `permeability` of `4π/αinv^2` and `electronmass` of `√(αG*αinv)`.
-
-$(textunits(Schrodinger,:Schrodinger))
-""" Schrodinger
-
-const Electronic = UnitSystem{1,αinv,1,4π,1}()
-@doc """
-    Electronic::UnitSystem{1,αinv,1,4π,1}
-
-Electronic `UnitSystem` with `planckreduced` of `αinv` and `permeability` of `4π`.
-
-$(textunits(Electronic,:Electronic))
-""" Electronic
-
-@doc """
-    Natural::UnitSystem{1,1,1,1,1}
-
-Natural `UnitSystem` with all primary constants having unit value.
-
-$(textunits(Natural,:Natural))
-
-The well known `Natural` values for `length`, `time`, `mass`, and `charge` are:
-```Julia
-julia> length(Natural,Metric)
-$(length(Natural,Metric))
-
-julia> time(Natural,Metric)
-$(time(Natural,Metric))
-
-julia> mass(Natural,Metric)
-$(mass(Natural,Metric))
-
-julia> charge(Natural,Metric)
-$(charge(Natural,Metric))
-```
-""" Natural
-
-@doc """
-    NaturalGauss::UnitSystem{1,1,1,4π,1}
-
-Natural (Gauss) `UnitSystem` with the Gaussian `permeability` value of `4π`.
-
-$(textunits(NaturalGauss,:NaturalGauss))
-""" NaturalGauss
-
-@doc """
-    QCD::UnitSystem{1,1,1,1,1/μₚₑ}
-
-Qunatum chromodynamics `UnitSystem` with `electronmass` of `1/μₚₑ` or `1/$μₚₑ`.
-
-$(textunits(QCD,:QCD))
-
-The well known `QCD` values for `length`, `time`, `mass`, and `charge` are:
-```Julia
-julia> length(QCD,Metric) # lQCD
-$(length(QCD,Metric))
-
-julia> time(QCD,Metric) # tQCD
-$(time(QCD,Metric))
-
-julia> mass(QCD,Metric) # mQCD
-$(mass(QCD,Metric))
-
-julia> charge(QCD,Metric)
-$(charge(QCD,Metric))
-```
-""" QCD
-
-@doc """
-    QCDGauss::UnitSystem{1,1,1,4π,1/μₚₑ}
-
-Qunatum chromodynamics (Gauss) `UnitSystem` with `electronmass` of `1/μₚₑ`.
-
-$(textunits(QCDGauss,:QCDGauss))
-
-The well known `QCDGauss` values for `length`, `time`, `mass`, and `charge` are:
-```Julia
-julia> length(QCDGauss,Metric) # lQCD
-$(length(QCDGauss,Metric))
-
-julia> time(QCDGauss,Metric) # tQCD
-$(time(QCDGauss,Metric))
-
-julia> mass(QCDGauss,Metric) # mQCD
-$(mass(QCDGauss,Metric))
-
-julia> charge(QCDGauss,Metric)
-$(charge(QCDGauss,Metric))
-```
-""" QCDGauss
-
-@doc """
-    QCDoriginal::UnitSystem{1,1,1,4π/αinv,1/μₚₑ}
-
-Qunatum chromodynamics (original) `UnitSystem` with `permeability` of `4π/αinv`.
-
-$(textunits(QCDoriginal,:QCDoriginal))
-
-The well known `QCDoriginal` values for `length`, `time`, `mass`, and `charge` are:
-```Julia
-julia> length(QCDoriginal,Metric) # lQCD
-$(length(QCDoriginal,Metric))
-
-julia> time(QCDoriginal,Metric) # tQCD
-$(time(QCDoriginal,Metric))
-
-julia> mass(QCDoriginal,Metric) # mQCD
-$(mass(QCDoriginal,Metric))
-
-julia> charge(QCDoriginal,Metric)
-$(charge(QCDoriginal,Metric))
-```
-""" QCDoriginal
+include("systems.jl")
 
 end # module
diff --git a/src/physics.jl b/src/physics.jl
index 4c6c545..6e58e9e 100644
--- a/src/physics.jl
+++ b/src/physics.jl
@@ -5,25 +5,25 @@
 @doc """
     μₑᵤ, μₚᵤ, μₚₑ, αinv, αG
 
-Physical measured dimensionless values with uncertainty are the electron to proton mass ratio `μₑᵤ`, proton to atomic mass ratio `μₚᵤ`, proton to electron mass ratio `μₚₑ`, inverted fine structure constant `αinv`, and the gravitaional coupling constant `αG`.
+Physical measured dimensionless `Coupling` values with uncertainty are the electron to proton mass ratio `μₑᵤ`, proton to atomic mass ratio `μₚᵤ`, proton to electron mass ratio `μₚₑ`, inverted fine structure constant `αinv`, and the gravitaional coupling constant `αG`.
 
 ```Julia
-julia> μₑᵤ
+julia> μₑᵤ # electronunit(Universe)
 $μₑᵤ
 
-julia> μₚᵤ
+julia> μₚᵤ # protonunit(Universe)
 $μₚᵤ
 
-julia> μₚₑ
+julia> μₚₑ # protonelectron(Universe)
 $μₚₑ
 
-julia> αinv
+julia> αinv # 1/finestructure(Universe)
 $αinv
 
-julia> αG
+julia> αG # coupling(Universe)
 $αG
 ```
-""" μₑᵤ, μₚᵤ, μₚₑ, αinv, αG, meu, mpu, mpe, ainv, aG
+""" Universe, μₑᵤ, μₚᵤ, μₚₑ, αinv, αG, meu, mpu, mpe, ainv, aG, electronunit, protonunit, protonelectron, finestructure, coupling
 
 @pure hyperfine(U::UnitSystem) = frequency(ΔνCs,U)
 @doc """
@@ -62,7 +62,7 @@ $(luminousefficacy(English))
 """ luminousefficacy, Kcd
 
 @doc """
-    molarmass(U::UnitSystem) = avogadro(U)*electronmass(U)/μₑᵤ # 1/μₑᵤ = $(1/μₑᵤ-2e-13)
+    molarmass(U::UnitSystem) = avogadro(U)*electronmass(U)/electronunit(U)
 
 Molar mass constant `Mᵤ` is the ratio of the `molarmass` and `relativemass` of a chemical.
 ```Julia
@@ -80,7 +80,7 @@ $(molarmass(SI2019))
 ```
 """ molarmass, Mᵤ, Mu
 
-@pure avogadro(U::UnitSystem) = μₑᵤ*molarmass(U)/electronmass(U)
+@pure avogadro(U::UnitSystem,C::Coupling=universe(U)) = molarmass(U,C)*electronunit(C)/electronmass(U,C)
 @doc """
     avogadro(x) = universal(x)/boltzmann(x) # Mᵤ/atomicmass(x), Mᵤ ≈ 0.001-3.5e-13
 
@@ -272,7 +272,7 @@ $(rationalization(Gauss))
 """ rationalization
 
 @doc """
-    electronmass(U::UnitSystem) = protonmass(U)/$μₚₑ # αinv^2*R∞*2𝘩/𝘤
+    electronmass(U::UnitSystem) = protonmass(U)/protonelectron(U) # αinv^2*R∞*2𝘩/𝘤
 
 Electron rest mass `mₑ` of subatomic particle with `-𝘦` elementary charge  (kg or slugs).
 ```Julia
@@ -293,7 +293,7 @@ $(electronmass(English))
 ```
 """ electronmass, mₑ, me
 
-@pure atomicmass(U::UnitSystem) = electronmass(U)/μₑᵤ
+@pure atomicmass(U::UnitSystem,C::Coupling=universe(U)) = electronmass(U,C)/electronunit(C)
 @doc """
     atomicmass(U::UnitSystem) = Mᵤ/avogadro(U) # $(molarmass(SI2019)) ≈ 0.001-3.5e-13
 
@@ -316,9 +316,9 @@ $(atomicmass(English))
 ```
 """ atomicmass, mᵤ, mu
 
-@pure protonmass(U::UnitSystem) =  μₚₑ*electronmass(U)
+@pure protonmass(U::UnitSystem,C::Coupling=universe(U)) = protonelectron(C)*electronmass(U,C)
 @doc """
-    protonmass(U::UnitSystem) = $(μₚᵤ)atomicmass(U)
+    protonmass(U::UnitSystem) = protonunit(U)*atomicmass(U)
 
 Proton mass `mₚ` of subatomic particle with `+𝘦` elementary charge  (kg or mass).
 ```Julia
@@ -337,7 +337,7 @@ $(protonmass(Metric)/electronmass(Metric))
 """ protonmass, mₚ, mp
 
 @doc """
-    planckmass(U::UnitSystem) = sqrt(planckreduced(U)*lightspeed(U)/newton(U))
+    planckmass(U::UnitSystem) = electronmass(U)/sqrt(coupling(U))
 
 Planck mass factor `mP` from the gravitational coupling constant `αG` (kg or slugs).
 ```Julia
@@ -371,7 +371,7 @@ $(newton(English))
 ```
 """ newton, GG
 
-@pure einstein(U::UnitSystem) = 8π*newton(U)/lightspeed(U)^4
+@pure einstein(U::UnitSystem,C::Coupling=universe(U)) = 8π*newton(U,C)/lightspeed(U,C)^4
 @doc """
     einstein(U::UnitSystem) = 8π*newton(U)/lightspeed(U)^4
 
@@ -382,7 +382,7 @@ $(einstein(Metric))
 ```
 """ einstein, κ
 
-@pure universal(U::UnitSystem) = boltzmann(U)*avogadro(U)
+@pure universal(U::UnitSystem,C::Coupling=universe(U)) = boltzmann(U,C)*avogadro(U,C)
 @doc """
     universal(x) = boltzmann(x)*avogadro(x)
 
@@ -414,7 +414,7 @@ $(universal(English))
 The 1976 United States Standard Atmosphere used R* = 8.31432 exactly.
 """ universal, Rᵤ, Ru
 
-@pure stefan(U::UnitSystem) = 2π^5*boltzmann(U)^4/(15planck(U)^3*lightspeed(U)^2)
+@pure stefan(U::UnitSystem,C::Coupling=universe(U)) = 2π^5*boltzmann(U,C)^4/(15planck(U,C)^3*lightspeed(U,C)^2)
 @doc """
     stefan(U::UnitSystem) = 2π^5*boltzmann(U)^4/(15planck(U)^3*lightspeed(U)^2)
 
@@ -451,9 +451,9 @@ julia> radiationdensity(English) # lb⋅ft⁻²⋅°R⁻⁴
 $(radiationdensity(English))
 ```
 """
-@pure radiationdensity(U::UnitSystem) = 4stefan(U)/lightspeed(U)
+@pure radiationdensity(U::UnitSystem,C::Coupling=universe(U)) = 4stefan(U,C)/lightspeed(U,C)
 
-@pure permittivity(U::UnitSystem) = inv(permeability(U)*(lightspeed(U)*lorentz(U))^2)
+@pure permittivity(U::UnitSystem,C::Coupling=universe(U)) = inv(permeability(U,C)*(lightspeed(U,C)*lorentz(U))^2)
 @doc """
     permittivity(U::UnitSystem) = 1/permeability(U)/(lightspeed(U)*lorentz(U))^2
 
@@ -483,7 +483,7 @@ $(permittivity(SI2019)/charge(SI2019))
 ```
 """ permittivity, ε₀, ϵ₀, e0
 
-@pure coulomb(U::UnitSystem) = rationalization(U)/4π/permittivity(U)
+@pure coulomb(U::UnitSystem,C::Coupling=universe(U)) = rationalization(U)/4π/permittivity(U,C)
 @doc """
     coulomb(U::UnitSystem) = rationalization(U)/4π/permittivity(U)
 
@@ -513,7 +513,7 @@ $(coulomb(HLU))
 ```
 """ coulomb, kₑ, ke
 
-@pure biotsavart(U::UnitSystem) = permeability(U)*lorentz(U)*(rationalization(U)/4π)
+@pure biotsavart(U::UnitSystem,C::Coupling=universe(U)) = permeability(U,C)*lorentz(U)*(rationalization(U)/4π)
 @doc """
     biotsavart(U::UnitSystem) = permeability(U)*lorentz(U)*rationalization(U)/4π
 
@@ -608,7 +608,7 @@ $(impedance(HLU))
 """ impedance, Z₀, Z0
 
 @doc """
-    charge(U::UnitSystem) = sqrt(2𝘩/$(αinv)impedance(U)) # faraday(U)/avogadro(U)
+    charge(U::UnitSystem) = √(2planck(U)*finestructure(U)/impedance(U)) # faraday(U)/avogadro(U)
 
 Quantized elementary charge `𝘦` of a proton or electron `2/(klitzing(U)*josephson(U))` (C).
 ```Julia
@@ -635,7 +635,7 @@ $(charge(Planck))
 ```
 """ charge, 𝘦, ee
 
-@pure faraday(U::UnitSystem) = charge(U)*avogadro(U)
+@pure faraday(U::UnitSystem,C::Coupling=universe(U)) = charge(U,C)*avogadro(U,C)
 @doc """
     faraday(U::UnitSystem) = charge(U)*avogadro(U)
 
@@ -667,7 +667,7 @@ $(faraday(Metric)/3600)
 ```
 """ faraday, 𝔉, FF
 
-@pure josephson(U::UnitSystem) = 2charge(U)*lorentz(U)/planck(U)
+@pure josephson(U::UnitSystem,C::Coupling=universe(U)) = 2charge(U,C)*lorentz(U)/planck(U,C)
 @doc """
     josephson(U::UnitSystem) = 2charge(U)*lorentz(U)/planck(U) # 1/magneticflux(U)
 
@@ -693,7 +693,7 @@ $(josephson(ESU))
 ```
 """ josephson, KJ
 
-@pure magneticflux(U::UnitSystem) = inv(josephson(U))
+@pure magneticflux(U::UnitSystem,C::Coupling=universe(U)) = inv(josephson(U,C))
 @doc """
     magneticflux(U::UnitSystem) = planck(U)/2charge(U)/lorentz(U)
 
@@ -716,7 +716,7 @@ $(magneticflux(ESU))
 ```
 """ magneticflux, Φ₀
 
-@pure klitzing(U::UnitSystem) = planck(U)/charge(U)^2
+@pure klitzing(U::UnitSystem,C::Coupling=universe(U)) = planck(U,C)/charge(U,C)^2
 @doc """
     klitzing(U::UnitSystem) = planck(U)/charge(U)^2
 
@@ -742,7 +742,7 @@ $(klitzing(ESU))
 ```
 """ klitzing, RK
 
-@pure conductance(U::UnitSystem) = 2charge(U)^2/planck(U)
+@pure conductance(U::UnitSystem,C::Coupling=universe(U)) = 2charge(U,C)^2/planck(U,C)
 @doc """
     conductance(U::UnitSystem) = 2charge(U)^2/planck(U) # 2/klitzing(U)
 
@@ -768,9 +768,9 @@ $(conductance(ESU))
 ```
 """ conductance, G₀, G0
 
-@pure hartree(U::UnitSystem) = electronmass(U)*(lightspeed(U)/αinv)^2
+@pure hartree(U::UnitSystem,C::Coupling=universe(U)) = electronmass(U,C)*(lightspeed(U,C)*finestructure(C))^2
 @doc """
-    hartree(U::UnitSystem) = electronmass(U)*(lightspeed(U)/$αinv)^2 # mₑ*(𝘤/αinv)^2
+    hartree(U::UnitSystem) = electronmass(U)*(lightspeed(U)*finestructure(U))^2 # mₑ*(𝘤/αinv)^2
 
 Hartree electric potential energy `Eₕ` of the hydrogen atom at ground state is `2R∞*𝘩*𝘤` (J).
 ```Julia
@@ -801,7 +801,7 @@ $(hartree(Metric)/boltzmann(Metric))
 In a Gaussian unit system where `4π*ε₀ == 1` the Hartree energy is `𝘦^2/a₀`.
 """ hartree, Eₕ, Eh
 
-@pure rydberg(U::UnitSystem) = hartree(U)/2planck(U)/lightspeed(U)
+@pure rydberg(U::UnitSystem,C::Coupling=universe(U)) = hartree(U,C)/2planck(U,C)/lightspeed(U,C)
 @doc """
     rydberg(U::UnitSystem) = hartree(U)/2planck(U)/lightspeed(U) # Eₕ/2𝘩/𝘤
 
@@ -839,9 +839,9 @@ $(1/rydberg(Metric)/2π)
 Precision measurements of the Rydberg constants are within a relative standard uncertainty of under 2 parts in 10¹², and is chosen to constrain values of other physical constants.
 """ rydberg, R∞, RH, Ry
 
-@pure bohr(U::UnitSystem) = αinv*planckreduced(U)/electronmass(U)/lightspeed(U)
+@pure bohr(U::UnitSystem,C::Coupling=universe(U)) = planckreduced(U,C)/electronmass(U,C)/lightspeed(U,C)/finestructure(C)
 @doc """
-    bohr(U) = $αinv*planckreduced(U)/electronmass(U)/lightspeed(U)
+    bohr(U) = planckreduced(U)/electronmass(U)/lightspeed(U)/finestructure(U)
 
 Bohr radius of the hydrogen atom in its ground state `a₀` (m).
 ```Julia
@@ -857,7 +857,7 @@ $(bohr(Metric)/length(PlanckGauss))
 """ bohr, a₀, a0
 
 """
-    bohrreduced(U::UnitSystem) = bohr(U)*(1+1/$μₚₑ)
+    bohrreduced(U::UnitSystem) = bohr(U)*(1+1/protonelectron(U))
 
 Reduced Bohr radius including the effect of reduced mass in hydrogen atom (m).
 ```Julia
@@ -868,11 +868,11 @@ julia> bohrreduced(Metric) # a₀
 $(bohrreduced(Metric)/bohr(Metric))
 ```
 """
-@pure bohrreduced(U::UnitSystem) = bohr(U)*(1+1/μₚₑ)
+@pure bohrreduced(U::UnitSystem,C::Coupling=universe(U)) = bohr(U,C)*(1+1/protonelectron(C))
 
-@pure electronradius(U::UnitSystem) = planckreduced(U)/electronmass(U)/lightspeed(U)/αinv
+@pure electronradius(U::UnitSystem,C::Coupling=universe(U)) = finestructure(C)*planckreduced(U,C)/electronmass(U,C)/lightspeed(U,C)
 @doc """
-    electronradius(U) = planckreduced(U)/electronmass(U)/lightspeed(U)/$αinv
+    electronradius(U) = finestructure(U)*planckreduced(U)/electronmass(U)/lightspeed(U)
 
 Classical electron radius or Lorentz radius or Thomson scattering length (m).
 ```Julia
@@ -887,7 +887,7 @@ $(electronradius(Conventional))
 ```
 """ electronradius, rₑ, re
 
-@pure magneton(U::UnitSystem) = charge(U)*planckreduced(U)*lorentz(U)/2electronmass(U)
+@pure magneton(U::UnitSystem,C::Coupling=universe(U)) = charge(U,C)*planckreduced(U,C)*lorentz(U)/2electronmass(U,C)
 """
     magneton(U::UnitSystem) = charge(U)*planckreduced(U)*lorentz(U)/2electronmass(U)
 
diff --git a/src/systems.jl b/src/systems.jl
new file mode 100644
index 0000000..28eb503
--- /dev/null
+++ b/src/systems.jl
@@ -0,0 +1,646 @@
+
+#   This file is part of UnitSystems.jl. It is licensed under the MIT license
+#   UnitSystems Copyright (C) 2021 Michael Reed
+
+export Universe, coupling, finestructure, electronunit, protonunit, protonelectron
+#const Mu,Ru,SB,hh,cc,m0,e0,ke,me,mp,mu,ee,FF,Z0,G0,Eh,a0,re,g0,lP,ϵ₀,mB = Mᵤ,Rᵤ,σ,𝘩,𝘤,μ₀,ε₀,kₑ,mₑ,mₚ,mᵤ,𝘦,𝔉,Z₀,G₀,Eₕ,a₀,rₑ,g₀,ℓP,ε₀,μB
+export slug, ft, KJ1990, KJ2014, RK1990, RK2014, mₑ1990, mₑ2014, temp, units
+export slugs, kilograms, lbm, meters, feet, rankine, kelvin, moles, molecules
+export UnitSystem, US, SI, MKS, CGS, CGS2019, CGSm, CGSe, HLU, FFF
+
+# == Metric is different
+const κ = einstein(SI2019)
+const σ = stefan(SI2019) #
+const μB = magneton(SI2019) #
+const ε₀ = permittivity(SI2019) #
+const kₑ = coulomb(SI2019) #
+const mₚ = protonmass(SI2019)
+const mᵤ = atomicmass(SI2019)
+const Mᵤ = molarmass(SI2019)
+const 𝔉 = faraday(SI2019) #
+const Φ₀ = magneticflux(SI2019) #
+const Z₀ = impedance(SI2019) #
+const G₀ = conductance(SI2019) #
+const Eₕ = hartree(SI2019)
+const a₀ = bohr(SI2019)
+const rₑ = electronradius(SI2019)
+const RK = klitzing(SI2019) #
+const KJ = josephson(SI2019) #
+const RH,Ry = R∞*mₚ/(mₑ+mₚ),𝘩*𝘤*R∞
+
+const ℓP = length(PlanckGauss,SI2019)
+const tP = time(PlanckGauss,SI2019)
+const TP = temperature(PlanckGauss,SI2019)
+
+const lS = length(Stoney,SI2019)
+const tS = time(Stoney,SI2019)
+const mS = mass(Stoney,SI2019)
+const qS = charge(Stoney,SI2019)
+
+const lA = length(Hartree,SI2019)
+const tA = time(Hartree,SI2019)
+const mA = mass(Hartree,SI2019)
+const qA = charge(Hartree,SI2019)
+
+const lQCD = length(QCD,SI2019)
+const tQCD = time(QCD,SI2019)
+const mQCD = mass(QCD,SI2019)
+
+# non standard units
+
+const BTUftlb = 3600/0.5778thermalconductivity(English) # BTU⋅ft⁻¹⋅lb⁻¹
+const BTUJ = energy(English)*BTUftlb # BTU⋅J⁻¹
+
+# constant aliases
+
+const mpe, meu, mpu, ainv, aG = μₚₑ, μₑᵤ, μₚᵤ, αinv, αG
+const Mu,Ru,SB,hh,cc,m0,e0,ke,me,mp,mu,ee,FF,Z0,G0,Eh,a0,re,g0,lP,aL,ϵ₀ = Mᵤ,Rᵤ,σ,𝘩,𝘤,μ₀,ε₀,kₑ,mₑ,mₚ,mᵤ,𝘦,𝔉,Z₀,G₀,Eₕ,a₀,rₑ,g₀,ℓP,αL,ε₀
+export κ, GG, NA, kB, Rᵤ, σ, 𝘩, ħ, 𝘤, μ₀, ε₀, kₑ, mₑ, mₚ, mᵤ, 𝘦, 𝔉, Φ₀, Z₀, G₀, Eₕ, R∞, a₀, rₑ, KJ, RK, Ru, SB, hh, cc, m0, e0, ke, me, mp, mu, ee, FF, Z0, G0, Eh, a0, re, μB
+export αG, αinv, μₚₑ, μₑᵤ, μₚᵤ, mpe, meu, mpu, mP, δμ₀, Mᵤ, Mu, RH, Ry, ΔνCs, Kcd, ainv
+export cal, kcal, calₜₕ, kcalₜₕ, calᵢₜ, kcalᵢₜ, ℓP, g₀, g0, atm, lbm, BTUJ, BTUftlb, aG
+export lP, tP, TP, lS, tS, mS, qS, lA, tA, mA, qA, lQCD, tQCD, mQCD, ϵ₀, αL, aL
+
+# engineering unit systems docs
+
+cgstext(US,AMP,cgs=eval(US)) = """
+```Julia
+julia> boltzmann($US) # erg⋅K⁻¹
+$(boltzmann(cgs))
+
+julia> planckreduced($US) # erg⋅s⋅rad⁻¹
+$(planckreduced(cgs))
+
+julia> lightspeed($US) # cm⋅s⁻¹
+$(lightspeed(cgs))
+
+julia> permeability($US) # statH⋅cm⁻¹
+$(permeability(cgs))
+
+julia> electronmass($US) # g
+$(electronmass(cgs))
+
+julia> rationalization($US)
+$(rationalization(cgs))
+```
+"""
+
+for U ∈ (:CGSm,:CGSe,:EMU,:ESU)
+    (EU,AMP) = QuoteNode.(U ∉ (:CGSe,:ESU) ? (:EMU,:Bi) : (:ESU,:statA))
+@eval @doc """
+    $($(QuoteNode(U)))::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,$($EU≠:EMU ? "(100𝘤)^-2" : 1),1000mₑ,4π}
+
+Centimetre-gram-second `UnitSystem` variant based on `$($EU)` (non-rationalized).
+
+$(cgstext($(QuoteNode(U)),$AMP))
+""" $U
+
+U ∉ (:CGSm,:CGSe) && @eval @doc """
+    $(Symbol($(QuoteNode(U)),:2019))::UnitSystem{1e7*kB,1e7*ħ,100𝘤,$($EU≠:EMU ? "1e3*μ₀/𝘤^2" : "1e7*μ₀"),1000mₑ}
+
+Centimetre-gram-second `UnitSystem` variant of tuned `SI2019` based on `$($EU)` (rationalized).
+
+$(cgstext(Symbol($(QuoteNode(U)),:2019),$AMP))
+""" $(Symbol(U,:2019))
+end
+
+@doc """
+    Thomson::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,4π,1/2}
+
+Centimetre-gram-second `UnitSystem` variant `Thomson` (EMU-Lorentz, non-rationalized).
+
+```Julia
+julia> boltzmann(Thomson) # erg⋅K⁻¹
+$(boltzmann(Thomson))
+
+julia> planckreduced(Thomson) # erg⋅s⋅rad⁻¹
+$(planckreduced(Thomson))
+
+julia> lightspeed(Thomson) # cm⋅s⁻¹
+$(lightspeed(Thomson))
+
+julia> permeability(Thomson) # abH⋅cm⁻¹
+$(permeability(Thomson))
+
+julia> electronmass(Thomson) # g
+$(electronmass(Thomson))
+
+julia> rationalization(Thomson)
+$(rationalization(Thomson))
+
+julia> lorentz(Thomson)
+$(lorentz(Thomson))
+```
+""" Thomson
+
+@doc """
+    Gauss::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,4π,0.01/𝘤}
+
+Centimetre-gram-second `UnitSystem` variant `CGS` (Gauss-Lorentz, non-rationalized).
+
+```Julia
+julia> boltzmann(Gauss) # erg⋅K⁻¹
+$(boltzmann(Gauss))
+
+julia> planckreduced(Gauss) # erg⋅s⋅rad⁻¹
+$(planckreduced(Gauss))
+
+julia> lightspeed(Gauss) # cm⋅s⁻¹
+$(lightspeed(Gauss))
+
+julia> permeability(Gauss) # statH⋅cm⁻¹
+$(permeability(Gauss))
+
+julia> electronmass(Gauss) # g
+$(electronmass(Gauss))
+
+julia> rationalization(Gauss)
+$(rationalization(Gauss))
+
+julia> lorentz(Gauss)
+$(lorentz(Gauss))
+```
+""" Gauss, CGS
+
+@doc """
+    LorentzHeaviside::UnitSystem{1e10*Rᵤ*mₑ/μₑᵤ,1e7*ħ,100𝘤,1,1000mₑ,1,0.01/𝘤}
+
+Centimetre-gram-second `UnitSystem` variant `HLU` (Heaviside-Lorentz, rationalized).
+
+```Julia
+julia> boltzmann(LorentzHeaviside) # erg⋅K⁻¹
+$(boltzmann(LorentzHeaviside))
+
+julia> planckreduced(LorentzHeaviside) # erg⋅s⋅rad⁻¹
+$(planckreduced(LorentzHeaviside))
+
+julia> lightspeed(LorentzHeaviside) # cm⋅s⁻¹
+$(lightspeed(LorentzHeaviside))
+
+julia> permeability(HLU) # hlH⋅cm⁻¹
+$(permeability(LorentzHeaviside))
+
+julia> electronmass(LorentzHeaviside) # g
+$(electronmass(LorentzHeaviside))
+
+julia> rationalization(LorentzHeaviside)
+$(rationalization(LorentzHeaviside))
+
+julia> lorentz(LorentzHeaviside)
+$(lorentz(LorentzHeaviside))
+```
+""" LorentzHeaviside, HLU
+
+@doc """
+    MTS::UnitSystem{1e6*Rᵤ*mₑ/μₑᵤ,1000ħ,𝘤,4π/1e4,mₑ/1000}
+
+Metre-tonne-second `UnitSystem` variant of `Metric` system.
+
+```Julia
+julia> boltzmann(MTS) # kJ⋅K⁻¹
+$(boltzmann(MTS))
+
+julia> planckreduced(MTS) # kJ⋅s⋅rad⁻¹
+$(planckreduced(MTS))
+
+julia> lightspeed(MTS) # m⋅s⁻¹
+$(lightspeed(MTS))
+
+julia> permeability(MTS) # kH⋅m⁻¹
+$(permeability(MTS))
+
+julia> electronmass(MTS) # t
+$(electronmass(MTS))
+```
+""" MTS
+
+@doc """
+    Metric::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001,ħ,𝘤,4π*1e-7,mₑ}
+
+Systeme International d'Unites (the SI units) adopted as the preferred `UnitSystem`.
+
+```Julia
+julia> boltzmann(Metric) # J⋅K⁻¹
+$(boltzmann(Metric))
+
+julia> planckreduced(Metric) # J⋅s⋅rad⁻¹
+$(planckreduced(Metric))
+
+julia> lightspeed(Metric) # m⋅s⁻¹
+$(lightspeed(Metric))
+
+julia> permeability(Metric) # H⋅m⁻¹
+$(permeability(Metric))
+
+julia> electronmass(Metric) # kg
+$(electronmass(Metric))
+```
+""" Metric
+
+@doc """
+    SI2019::UnitSystem{kB,ħ,𝘤,μ₀,mₑ}
+
+Systeme International d'Unites (the SI units) with `μ₀` for a tuned `charge` exactly.
+
+```Julia
+julia> boltzmann(SI2019) # J⋅K⁻¹
+$(boltzmann(SI2019))
+
+julia> planckreduced(SI2019) # J⋅s⋅rad⁻¹
+$(planckreduced(SI2019))
+
+julia> lightspeed(SI2019) # m⋅s⁻¹
+$(lightspeed(SI2019))
+
+julia> permeability(SI2019) # H⋅m⁻¹
+$(permeability(SI2019))
+
+julia> electronmass(SI2019) # kg
+$(electronmass(SI2019))
+```
+""" SI2019, SI
+
+@doc """
+    CODATA::UnitSystem{Rᵤ*mₑ2014/μₑᵤ/0.001,2/RK2014/KJ2014^2/π,𝘤,2RK2014/𝘤/αinv,mₑ2014}
+
+Metric `UnitSystem` based on Committee on Data of the International Science Council.
+
+```Julia
+julia> boltzmann(CODATA) # J⋅K⁻¹
+$(boltzmann(CODATA))
+
+julia> planckreduced(CODATA) # J⋅s⋅rad⁻¹
+$(planckreduced(CODATA))
+
+julia> lightspeed(CODATA) # m⋅s⁻¹
+$(lightspeed(CODATA))
+
+julia> permeability(CODATA) # H⋅m⁻¹
+$(permeability(CODATA))
+
+julia> electronmass(CODATA) # kg
+$(electronmass(CODATA))
+```
+""" CODATA
+
+@doc """
+    Conventional::UnitSystem{Rᵤ*mₑ1990/μₑᵤ/0.001,2/RK1990/KJ1990^2/π,𝘤,2RK1990/𝘤/αinv,mₑ1990}
+
+Conventional electronic `UnitSystem` with 1990 tuned `josephson` and `klitzing` constants.
+
+```Julia
+julia> boltzmann(Conventional) # J⋅K⁻¹
+$(boltzmann(Conventional))
+
+julia> planckreduced(Conventional) # J⋅s⋅rad⁻¹
+$(planckreduced(Conventional))
+
+julia> lightspeed(Conventional) # m⋅s⁻¹
+$(lightspeed(Conventional))
+
+julia> permeability(Conventional) # H⋅m⁻¹
+$(permeability(Conventional))
+
+julia> electronmass(Conventional) # kg
+$(electronmass(Conventional))
+```
+""" Conventional
+
+@doc """
+    IAU::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001/Jₛ,ħ*day/Jₛ,day*𝘤/au,4π*1e-7*day^2/Jₛ,mₑ/mₛ}
+
+Astronomical (solar) `UnitSystem` defined by International Astronomical Union.
+
+```Julia
+julia> boltzmann(IAU) # M⊙⋅au²⋅D⁻²⋅K⁻¹
+$(boltzmann(IAU))
+
+julia> planckreduced(IAU) # M⊙⋅au²⋅D⁻¹⋅rad⁻¹
+$(planckreduced(IAU))
+
+julia> lightspeed(IAU) # au⋅D⁻¹
+$(lightspeed(IAU))
+
+julia> permeability(IAU) # M⊙⋅au²⋅C⁻²
+$(permeability(IAU))
+
+julia> electronmass(IAU) # M⊙
+$(electronmass(IAU))
+```
+""" IAU
+
+@doc """
+    English::UnitSystem{kB*rankine/slug/ft^2,ħ/slug/ft^2,𝘤/ft,4π,mₑ/slug}
+
+Engineering `UnitSystem` historically used by Britain and United States.
+
+```Julia
+julia> boltzmann(English) # ft⋅lb⋅°R⁻¹
+$(boltzmann(English))
+
+julia> planckreduced(English) # ft⋅lb⋅s⋅rad⁻¹
+$(planckreduced(English))
+
+julia> lightspeed(English) # ft⋅s⁻¹
+$(lightspeed(English))
+
+julia> permeability(English) # slug⋅ft²⋅?⁻²
+$(permeability(English))
+
+julia> electronmass(English) # slugs
+$(electronmass(English))
+```
+""" English
+
+@doc """
+    EnglishUS::UnitSystem{1000Rᵤ*mₑ/μₑᵤ*rankine/slug/ftUS^2,ħ/slug/ftUS^2,𝘤/ftUS,4π,mₑ/slug}
+
+Engineering `UnitSystem` based on the geophysical US survey foot (1200/3937).
+
+```Julia
+julia> boltzmann(EnglishUS) # ftUS⋅lb⋅°R⁻¹
+$(boltzmann(EnglishUS))
+
+julia> planckreduced(EnglishUS) # ftUS⋅lb⋅s⋅rad⁻¹
+$(planckreduced(EnglishUS))
+
+julia> lightspeed(EnglishUS) # ftUS⋅s⁻¹
+$(lightspeed(EnglishUS))
+
+julia> permeability(EnglishUS) # slug⋅ftUS²⋅?⁻²
+$(permeability(EnglishUS))
+
+julia> electronmass(EnglishUS) # slugs
+$(electronmass(EnglishUS))
+```
+""" EnglishUS
+
+@doc """
+    FFF::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001*rankine/Jf,ħ/14day/Jf,14day*𝘤/201.168,0,mₑ/mf}
+
+Furlong–firkin–fortnight `FFF` is a humorous `UnitSystem` based on unusal impractical units.
+
+```Julia
+julia> boltzmann(FFF) # fir⋅fur²⋅ftn⁻²⋅F⁻¹
+$(boltzmann(FFF))
+
+julia> planckreduced(FFF) # fir⋅fur²⋅ftn⁻¹⋅rad⁻¹
+$(planckreduced(FFF))
+
+julia> lightspeed(FFF) # fur⋅ftn⁻¹
+$(lightspeed(FFF))
+
+julia> permeability(FFF) # fir⋅fur²⋅Inf⁻²
+$(permeability(FFF))
+
+julia> electronmass(FFF) # fir
+$(electronmass(FFF))
+```
+""" FFF
+
+@doc """
+    Kennelly::UnitSystem{Rᵤ*mₑ/μₑᵤ/0.001,ħ,𝘤,1e-7,mₑ,4π}
+
+Kennelly ? variant `UnitSystem` of the standard `Metric` units ???
+
+```Julia
+julia> boltzmann(Kennelly) # J⋅K⁻¹
+$(boltzmann(Kennelly))
+
+julia> planckreduced(Kennelly) # J⋅s⋅rad⁻¹
+$(planckreduced(Kennelly))
+
+julia> lightspeed(Kennelly) # m⋅s⁻¹
+$(lightspeed(Kennelly))
+
+julia> permeability(Kennelly) # H⋅m⁻¹
+$(permeability(Kennelly))
+
+julia> electronmass(Kennelly) # kg
+$(electronmass(Kennelly))
+
+julia> rationalization(Kennelly)
+$(rationalization(Kennelly))
+```
+""" Kennelly
+
+# natural unit system docs
+
+textunits(U,S) = """
+```Julia
+julia> boltzmann($S)
+$(boltzmann(U))
+
+julia> planckreduced($S)
+$(planckreduced(U))
+
+julia> lightspeed($S)
+$(lightspeed(U))
+
+julia> permeability($S)
+$(permeability(U))
+
+julia> electronmass($S)
+$(electronmass(U))
+```
+"""
+
+@doc """
+    Planck::UnitSystem{1,1,1,1,√(4π*αG)}
+
+Planck `UnitSystem` with the `electronmass` value `√(4π*αG)` using gravitational coupling.
+
+$(textunits(Planck,:Planck))
+""" Planck
+
+@doc """
+    PlanckGauss::UnitSystem{1,1,1,4π,√αG}
+
+Planck (Gauss) `UnitSystem` with `permeability` of `4π` and `electronmass` coupling `√αG`.
+
+$(textunits(PlanckGauss,:PlanckGauss))
+
+The well known `PlanckGauss` values for `length`, `time`, `mass`, and `temperature` are:
+```Julia
+julia> length(PlanckGauss,Metric) # ℓP
+$(length(PlanckGauss,Metric))
+
+julia> time(PlanckGauss,Metric) # tP
+$(time(PlanckGauss,Metric))
+
+julia> mass(PlanckGauss,Metric) # mP
+$(mass(PlanckGauss,Metric))
+
+julia> temperature(PlanckGauss,Metric) # TP
+$(temperature(PlanckGauss,Metric))
+```
+""" PlanckGauss
+
+@doc """
+    Stoney::UnitSystem{1,αinv,1,4π,√(αG*αinv)}
+
+Stoney `UnitSystem` with `permeability` of `4π` and `electronmass` coupling `√(αG*αinv)`.
+
+$(textunits(Stoney,:Stoney))
+
+The well known `Stoney` values for `length`, `time`, `mass`, and `charge` are:
+```Julia
+julia> length(Stoney,Metric) # lS
+$(length(Stoney,Metric))
+
+julia> time(Stoney,Metric) # tS
+$(time(Stoney,Metric))
+
+julia> mass(Stoney,Metric) # mS
+$(mass(Stoney,Metric))
+
+julia> charge(Stoney,Metric) # qS
+$(charge(Stoney,Metric))
+```
+""" Stoney
+
+@doc """
+    Hartree::UnitSystem{1,1,αinv,4π/αinv^2,1}
+
+Hartree atomic `UnitSystem` with `lightspeed` of `αinv` and `permeability` of `4π/αinv^2`.
+
+$(textunits(Hartree,:Hartree))
+
+The well known `Hartree` atomic unit values for `length`, `time`, `mass`, and `charge` are:
+```Julia
+julia> length(Hartree,Metric) # lA
+$(length(Hartree,Metric))
+
+julia> time(Hartree,Metric) # tA
+$(time(Hartree,Metric))
+
+julia> mass(Hartree,Metric) # mA
+$(mass(Hartree,Metric))
+
+julia> charge(Hartree,Metric) # qA
+$(charge(Hartree,Metric))
+```
+""" Hartree
+
+@doc """
+    Rydberg::UnitSystem{1,1,2αinv,π/αinv^2,1/2}
+
+Rydberg `UnitSystem` with `lightspeed` of `2αinv` and `permeability` of `π/αinv^2`.
+
+$(textunits(Rydberg,:Rydberg))
+""" Rydberg
+
+@doc """
+    Schrodinger::UnitSystem{1,1,αinv,4π/αinv^2,√(αG*αinv)}
+
+Schrodinger `UnitSystem` with `permeability` of `4π/αinv^2` and `electronmass` of `√(αG*αinv)`.
+
+$(textunits(Schrodinger,:Schrodinger))
+""" Schrodinger
+
+@doc """
+    Electronic::UnitSystem{1,αinv,1,4π,1}
+
+Electronic `UnitSystem` with `planckreduced` of `αinv` and `permeability` of `4π`.
+
+$(textunits(Electronic,:Electronic))
+""" Electronic
+
+@doc """
+    Natural::UnitSystem{1,1,1,1,1}
+
+Natural `UnitSystem` with all primary constants having unit value.
+
+$(textunits(Natural,:Natural))
+
+The well known `Natural` values for `length`, `time`, `mass`, and `charge` are:
+```Julia
+julia> length(Natural,Metric)
+$(length(Natural,Metric))
+
+julia> time(Natural,Metric)
+$(time(Natural,Metric))
+
+julia> mass(Natural,Metric)
+$(mass(Natural,Metric))
+
+julia> charge(Natural,Metric)
+$(charge(Natural,Metric))
+```
+""" Natural
+
+@doc """
+    NaturalGauss::UnitSystem{1,1,1,4π,1}
+
+Natural (Gauss) `UnitSystem` with the Gaussian `permeability` value of `4π`.
+
+$(textunits(NaturalGauss,:NaturalGauss))
+""" NaturalGauss
+
+@doc """
+    QCD::UnitSystem{1,1,1,1,1/μₚₑ}
+
+Qunatum chromodynamics `UnitSystem` with `electronmass` of `1/μₚₑ` or `1/$μₚₑ`.
+
+$(textunits(QCD,:QCD))
+
+The well known `QCD` values for `length`, `time`, `mass`, and `charge` are:
+```Julia
+julia> length(QCD,Metric) # lQCD
+$(length(QCD,Metric))
+
+julia> time(QCD,Metric) # tQCD
+$(time(QCD,Metric))
+
+julia> mass(QCD,Metric) # mQCD
+$(mass(QCD,Metric))
+
+julia> charge(QCD,Metric)
+$(charge(QCD,Metric))
+```
+""" QCD
+
+@doc """
+    QCDGauss::UnitSystem{1,1,1,4π,1/μₚₑ}
+
+Qunatum chromodynamics (Gauss) `UnitSystem` with `electronmass` of `1/μₚₑ`.
+
+$(textunits(QCDGauss,:QCDGauss))
+
+The well known `QCDGauss` values for `length`, `time`, `mass`, and `charge` are:
+```Julia
+julia> length(QCDGauss,Metric) # lQCD
+$(length(QCDGauss,Metric))
+
+julia> time(QCDGauss,Metric) # tQCD
+$(time(QCDGauss,Metric))
+
+julia> mass(QCDGauss,Metric) # mQCD
+$(mass(QCDGauss,Metric))
+
+julia> charge(QCDGauss,Metric)
+$(charge(QCDGauss,Metric))
+```
+""" QCDGauss
+
+@doc """
+    QCDoriginal::UnitSystem{1,1,1,4π/αinv,1/μₚₑ}
+
+Qunatum chromodynamics (original) `UnitSystem` with `permeability` of `4π/αinv`.
+
+$(textunits(QCDoriginal,:QCDoriginal))
+
+The well known `QCDoriginal` values for `length`, `time`, `mass`, and `charge` are:
+```Julia
+julia> length(QCDoriginal,Metric) # lQCD
+$(length(QCDoriginal,Metric))
+
+julia> time(QCDoriginal,Metric) # tQCD
+$(time(QCDoriginal,Metric))
+
+julia> mass(QCDoriginal,Metric) # mQCD
+$(mass(QCDoriginal,Metric))
+
+julia> charge(QCDoriginal,Metric)
+$(charge(QCDoriginal,Metric))
+```
+""" QCDoriginal
diff --git a/test/runtests.jl b/test/runtests.jl
index 66ddee4..8e0a8c5 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -71,7 +71,7 @@ for S ∈ UnitSystems.Systems
                 @test electronmass(U) ≈ planckmass(U)*sqrt(αG)
             end
             @testset "hartree" begin
-                @test hartree(U) == electronmass(U)*(lightspeed(U)/αinv)^2
+                @test hartree(U) ≈ electronmass(U)*(lightspeed(U)/αinv)^2
                 @test hartree(U) ≈ planckreduced(U)*lightspeed(U)/αinv/bohr(U)
                 @test hartree(U) ≈ planckreduced(U)^2/electronmass(U)/bohr(U)^2
                 @test hartree(U) ≈ 2rydberg(U)*planck(U)*lightspeed(U)
@@ -161,7 +161,7 @@ for S ∈ UnitSystems.Systems
                 @test coulomb(U) ≈ permeability(U)*rationalization(U)*(lorentz(U)*lightspeed(U))^2/4π
                 @test coulomb(U) ≈ planckreduced(U)*lightspeed(U)/αinv/charge(U)^2
                 @test coulomb(U) ≈ klitzing(U)*lightspeed(U)/αinv/2π
-                @test coulomb(U) == biotsavart(U)/lorentz(U)/permeability(U)/permittivity(U)
+                @test coulomb(U) ≈ biotsavart(U)/lorentz(U)/permeability(U)/permittivity(U)
                 @test coulomb(U) ≈ ampere(U)*lightspeed(U)^2
             end
             @testset "ampere" begin
@@ -173,7 +173,7 @@ for S ∈ UnitSystems.Systems
             end
             @testset "lorentz" begin
                 @test lorentz(U) ≈ 1/lightspeed(U)/sqrt(permeability(U)*permittivity(U))
-                @test lorentz(U) == biotsavart(U)/permeability(U)/permittivity(U)/coulomb(U)
+                @test lorentz(U) ≈ biotsavart(U)/permeability(U)/permittivity(U)/coulomb(U)
                 @test lorentz(U) ≈ 4π*biotsavart(U)/rationalization(U)/permeability(U)
                 @test lorentz(U) == ampere(U)/biotsavart(U)
             end