Letterforms, and the making of pages
Medieval scribes placed a text block by drawing nine lines and reading the answer off the diagonals. No measuring, no arithmetic, no taste required — and the proportions that fall out are still the best default anybody has for a page of running text.
Nine roles, one ratio, and no size typed twice. Every measurement on the page comes from what a piece of text IS rather than from where it happens to sit, so one setting changes the whole page instead of one heading.
Sixty-six characters is not a superstition. It is the width at which the eye still finds the start of the next line without hunting for it, and it decides more about a reading page than the choice of typeface does.
A scribe ruled the page before writing a word, and the ruling was arithmetic: nib widths in, line height out. Every margin on this site comes from the same calculation, done by a stylesheet instead of a lead point.