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 `i`^{2} = -1

.

In italics is the speed of light, *c*

, the Avogadro constant, *N*_{A}

.
- Well-known functions are
**upright**, like `sin `*x*

or `exp `*x*

.

And this means, yes, also the the `e`

in `e`^{2x}

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). `d`*x*/d*t*

.
- 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*/MPa

^{1}. 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: