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 (
Kfor Kelvin), small when not (
mfor 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.
πis upright, so is
i2 = -1.
In italics is the speed of light,
c, the Avogadro constant,
- Well-known functions are upright, like
And this means, yes, also the the
e2xis 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).
- Particle name abbreviations are upright. Independently if it’s a latin letter (the electron’s
e) or a greek one (the pion’s
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.
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
Ressources I used to compile this list, sorted by importance:
- ISO 80000-2 standard
- NIST’s »Typefaces for Symbols in Scientific Manuscripts« handout (careful: a little old)
- Bureau International des Poids et Mesures, The International System of Units (SI), Brochure, 2008, Section 5.3
- A handy Stackexchange post, and another blog post with pictures.
Think about it: Every quantity
ρon the axis is divided by the unit, hence the number essentially loses its dimension, as shown on the axis. ↩