My "general" question is:
- Why do different fonts have different definitions of a "point"?
Or in a concrete case:
- Why is Arial 12pt bigger than Times New Roman 12pt?
The fact the definition of point size changes from one font to another confuses me.
General or theoretical answers are welcome.
PS: Although this question is not directly (La)TeX-related, at the same time it is, because, of course, (La)TeX uses fonts.
EDIT AND POSSIBLE ANSWER: Here, a sample document with words of 20 repeated characters of each latin character, in Times (upper line) and Arial (just below it):
We can see that, 50% of letters, specially vowels, are bigger in Arial, 35% of similar size, and 15% are bigger in Times. But these differences in size are only real in width, since it changes the number of characters which can fit in a line (and in a page).
About the vertical space consumed by both fonts, the x-height of Arial is bigger than Times (the x-height is which have in common the letters: pb), but the size of the x-height + ascenders (which have in common bh) or x-height + descender (which have in common pq) are the same in both fonts.
In general, the "point size" of a font is the size of the x-height + ascender area's size + descender area's size, and the point size says nothing and gives no restrictions about the width of each character. So, the only reason Arial is bigger than Times is because of its width. Any other size difference depends only on the visual style of the font design (in this case, the fact Arial has a bigger x-height).
Finally, how large is a point? does every font use the same definition of "point size"? In principle, the size of a point is 0.35278 mm, and thus 12pt (a pica), 12*0.35278 = 4.23333 mm. This point size is supposed to be a standard one, and it seems both Arial and Times New Roman uses this definition of point (tested with a rule in the screen :P). Although, in general, I don't know if, really, every modern font respect it and, according to some of the comments and other Internet sources, this seems not to be the case.