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In LaTeX, you can easily change the font size, using the \fontsize macro (described here) followed by the \selectfont macro. But what is this font size? I changed the font size using the code

\fontsize{10cm}{1.2em}\selectfont

and wrote Mx, and then printed it on my laser printed. I then measured the width of the M and the height of the x; these measured turned out to be approximately 6.9 cm and 3.9 cm, respectively. So none of these turned out to be 10 cm. Not even the diagonal of the M measured 10 cm, even if it wasn't far from it; this measured 9.4 cm.

I then printed the text lql, drew a straight line between the tops of the two ls, and measured the distance from the bottom of the q to the line I had just drawn, thinking that this distance might turn out to be 10 cm. Nope. Only 8.2 cm.

So what measure is it that is 10 cm, that is, where in the text do I find the font size? I want to know this because I need to adjust the vertical distance between the lowest descender and the highest ascender to 0.5 cm on a text I'm going to have at the spine of a book.

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    This may be because you're not loading a font that is scalable to the desired height. For example, are you loading lmodern? Are you receiving any warnings about font substitutions in your .log?
    – Werner ♦
    Commented Jan 17, 2014 at 17:56
  • Yes, I'm using the lmodern package, and the compilation does not generate any warning or error.
    – StrawberryFieldsForever
    Commented Jan 17, 2014 at 18:33

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Generally speaking lead type, the traditional means of printing, consists of individual rectangular pieces of lead with one raised letter at the end. These were assembled into lines, ink applied to the type and then pressed against a piece of paper to obtain the final printed document. The size of a fount is the height of the face of the lead blocks bearing the letters. Of necessity, this must be larger than any of the letters (i.e. ascenders + descenders).

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I hope it's ok to be super lazy today and just quote a paragraph from https://en.wikipedia.org/wiki/Typeface in full:

The size of typefaces and fonts is traditionally measured in points; point has been defined differently at different times, but now the most popular is the Desktop Publishing point of 1⁄72 in (0.0139 in or 0.35 mm). When specified in typographic sizes (points, kyus), the height of an em-square, an invisible box which is typically a bit larger than the distance from the tallest ascender to the lowest descender, is scaled to equal the specified size. For example, when setting Helvetica at 12 point, the em square defined in the Helvetica font is scaled to 12 points or 1⁄6 in (0.17 in or 4.3 mm). Yet no particular element of 12-point Helvetica need measure exactly 12 points.

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  • I thought that em was roughly the width of an 'M' (uppercase) in the current font? (At least it is according to Wikibooks/LaTeX/Lengths) But my M is only 6.9 cm wide, and it seems to scale properly when I change the font size from 10 cm to 11 cm as well.
    – StrawberryFieldsForever
    Commented Jan 17, 2014 at 18:31
  • Aren't those what TeX calls big points (bp) as opposed to (printers') points (pt)? I read an explanation of this recently in the manual for a package but am afraid I can't now remember which one. (Koma? memoir?)
    – cfr
    Commented Jan 18, 2014 at 0:56
  • @cfr Yes, postscript points, called desktop publishing points here, are called big points in TeX.
    – Christian
    Commented Jan 18, 2014 at 11:18
  • @StrawberryFieldsForever Did you measure how wide 1em was?
    – Christian
    Commented Jan 18, 2014 at 11:19
  • @Christian: What do you mean how wide 1em is? em is a length unit so it doesn't have a width. But I measured the width of an M, which I wrote in my last comment, if that is what you mean.
    – StrawberryFieldsForever
    Commented Jan 18, 2014 at 17:07

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