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It has been proposed that mathematical constants should be set upright to distinguish them from variables.1 This rule can also be stated as:2

There's a little-known rule, best stated as

“Operators and constants with a fixed meaning should be set upright.”

This results in more readable equations overall, and adds clarity:2

Comparison of upright and italic constants

However, I have my doubts as to how far this rule should extend. It is obvious to typeset π, i, or e upright. But should I also typeset the physical constants that I use in my publications upright? Where should one draw the line? Can the simple "little-known rule" mentioned above be extended?


[1]: ISO 80000-2
[2]: J. Küster. Math never seen. TUGboat, 31(2):221–229, 2010.

closed as off-topic by egreg, Peter Jansson, Svend Tveskæg, Jesse, Thorsten Feb 2 '14 at 13:09

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question does not fall within the scope of TeX, LaTeX or related typesetting systems as defined in the help center." – egreg, Peter Jansson, Svend Tveskæg, Jesse, Thorsten
If this question can be reworded to fit the rules in the help center, please edit the question.

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    This is just a comment: if the physical constant has a subscript like \lambda_\textup{C} then there is no need. In case of i, I use \matrm{i} to differentiate it from, e.g., \sum_i, and, in case of the upright π (the constant), to differentiate it from functions/proyections usually denoted with \pi(x). In those cases, if you see it upright your mind doesn't even think that it could be an index/function. – Manuel Feb 2 '14 at 11:14
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    ISO prescribes upright type for mathematical constants and italics for physical constants; I have never understood what's the difference. Probably they tossed a coin because of Euler's constant and the electron charge, and math won the upright type. The usage of upright "i" and "e" is not very spread in mathematical papers and books, it is in physics, engineering and related fields. But the question is off topic. – egreg Feb 2 '14 at 11:29
  • @egreg I'd say the fundamental charge is upright too, as it's an electron :-) – Joseph Wright Feb 2 '14 at 11:40
  • I don't see where this could be “on-topic”, since, apart from LaTeX, I don't usually differentiate between italics and upright in handwritten math. – Manuel Feb 2 '14 at 15:25
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    It would be nice if when a question is put on hold or closed, the act would be accompanied by a recommendation where else the user could try to get help. Besides, I think that there are questions that are more off-topic than this one. Nevertheless I accept the decision by the community. – Ingo Feb 2 '14 at 16:10
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The comprehensive list is in section 1.6 of Quantities, Units and Symbols in Physical Chemistry (aka 'IUPAC Green Book'). It says physical quantities should be typeset in italic whereas mathematical constants in upright. It also says physical constants should be typeset in italic because they are 'regarded as quantities subject to measurement'.

  • I like this point. – Manuel Feb 2 '14 at 11:43
  • That justification has always perplexed me: either the speed of light is constant or it isn't. If it is, the fact we're not able to express its value in a "mathematically precise" way is irrelevant. After all, when a computation is involved, an approximate value of pi is used, which is the same situation as with the speed of light. – egreg Feb 2 '14 at 12:05

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