When to use upright letters in math mode

what are the conventions when to use upright letters in math mode?

Example:

    \begin{align}
W_{mag} = \frac{1}{2}LI^2
\end{align}


should the inductivity and the current be typeset in upright letters (\mathrm)?

• Welcome to TeX.SX! While this is an interesting question, it's not really about TeX, but rather about style. – egreg Aug 2 '15 at 16:17
• @egreg Would it be better asked on Maths or Physics SE? (I don't know if this is discipline dependent or not?) – cfr Aug 2 '15 at 16:19
• Possibly Physics.SE, if the topic is physics, but I'm not sure if they accept such questions. Anyway, a subscript should be upright if it is verbal (a word or abbreviation thereof) as opposed to a symbol. – egreg Aug 2 '15 at 16:20
• @egreg: I am not very active on Physics.SE, but I doubt they would accept this question, since it's not about Physics itself, 'only' formatting. Of course, the index quantity should be upright if it's an abbreviation or a word, and just as usual if it's a symbol – user31729 Aug 2 '15 at 17:54
• Could Graphic Design SX be an alternative? There should be professional typesetters around that may also have experience with mathematics and physics print media. – Ruben Aug 2 '15 at 19:15

$W_\mathrm{mag} = \frac{1}{2} L I^2$