I researched on how to write units, constants and quantities, and variables the proper way. There’s a lot of information out in the interwebs. And, of course, not everything is correct.
In 2010, a new ISO standard was published, the ISO 80000-2, describing mathematical signs and symbols. You can buy it here (buying a ISO paper, it’s strange, right?).
There’s a LaTeX package called isomath implementing this standard for your favorite typesetting tool.
Following some notes from it on upright-edness and italic-ity.
- Units are to be written upright (roman). Capital, when they are based on names (
K for Kelvin), small when not (m for meter). See more on that at Andrés blog.
- Variables are to be written in italics (slanted), independently if the character is latin or greek.
x = 12, α = 1.
- Mathematical constants are to be written upright. Physical ›constants‹ in italics.
π is upright, so is i as in i2 = -1.
In italics is the speed of light, c, the Avogadro constant, NA.
- Well-known functions are upright, like
sin x or exp x.
And this means, yes, also the the e in e2x is upright, as it’s both a function and a mathematical constant.
- Indices of variables are upright when they are descriptive. See the Avogadro constant above.
- Differential operators are upright, but not the variables/quantities they differentiate (for).
dx/dt.
- Particle name abbreviations are upright. Independently if it’s a latin letter (the electron’s
e) or a greek one (the pion’s π).
While we are at it: The correct way to describe axes in plots is to state the measured variable (e.g. ρ), state a slash as a division sign (/) and then the unit of the variable (e.g. MPa) – ρ/MPa1. The order of variable and unit can be interchanged if the value given is reciprocal, e.g. 1/ρ would lead to a description of MPa/ρ. Writing square brackets for the unit is not correct (as in [MPa]).
Ressources I used to compile this list, sorted by importance:
