Font measurements are given in terms of the "em square", a notional square inside which each glyph sits (this is why the CSS "em" unit has "1em" equalling "the

Font measurements are given in terms of the "em square", a notional square inside which each glyph sits (this is why the CSS "em" unit has "1em" equalling "the current font size"). A 12px font should have each glyph fit within the same 12×12px square. Exactly where the characters are drawn in that square, and at what relative size, is up to the designer - but usually they're drawn so that the em covers the distance from the bottom of lower-case descenders (j, p, q, y) to the top of ascenders (b, d, k, l). Historically, in the days when a "font" meant "a set of metal blocks of uniform size with a glyph engraved on the end of each one", the "font size" was the height of each block of metal, which was an important thing to know when all the metal blocks used to set a page of type had to physically sit next to each other.