# [Tex/LaTex] Should subscripts in math mode be upright

italicmath-modetypography

In one-letter subscripts, like $x_i$, the subscript is usually typeset italic.

Is this the typographically right thing to do?
Consider this case:

The Boltzmann constant is often written as $k_B$, which is short for $k_{Boltzmann}$. But the lengthy Boltzmann looks strange if typeset all italic. So I usually write $k_{\mathrm{Boltzmann}}$.

But shouldn't then, for consistency, the B in $k_B$ also be upright? And how about the i in $x_i$?

Using "all italics" is unfortunately an often committed sin. You should italicize only variables.

Everything else should be upright. For example:

• function names (sin, cos, log, ln etc...)
• dimensionless numbers (Re, Pr, Ra...)
• exact infinitesimal increments (dx, dy et... in BOTH integrals and differentials)
• descriptive text
• all descriptive variable indices (unless they are also variables)

Exceptions to this rule may still apply, i.e. the Euler-number $e$ is no variable, but still traditionally written in italics.

Some of the above was stolen from the "Please Make A Note" blog. More detailed information can be found in "Typefaces" from NIST.