[Tex/LaTex] Why LaTeX uses upright uppercase Greek letters by default in math mode

capitalizationfontsgreekmath-mode

Why upper-case Greek letters are typeset in upright font, i.e., \mathrm? While at the same time, some upper-case Greek letters those are also Latin Letters ,e.g., A for $\Alpha$, B for $\Beta$, are typeset in \mathnormal font, which leads to inconsistency? Can anyone give some explanations?

From the "Short math guide for LaTeX" by AMS, I've found:

For obscure historical reasons, the default font for lowercase Greek
letters in math formulas is italic while the default font for capital
Greek letters is upright/roman.

So what are the historical reasons?

Best Answer

Computer Modern's greek (bottom) font is (loosely) based on Monotype's 155M Greek font (top):

comparison between CM and Monotype Greek font

The Monotype 155M font itself is related to the Porson greek font, which was one of the most used typeface for greek in english speaking countries. One of the characteristic of this font is that it has upright capitals but slanted lowercase. This might seem surprising today, but the first italic fonts all had upright capitals (see for example the image of the Aldine Press article on wikipedia)

Notice that in France, the traditional font for greek was Didot's, which has both upright capitals and lowercase, which is why using upright lowercase greek was standard in France.

You can see versions of the Didot and Porson fonts on the Greek Font Society website.

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