The measure is the design
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.
Series: Designing a reading page
A type scale you can defend
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.
Optical size, and why 8pt is not 24pt shrunk
A caption and a headline want different letterforms, not the same letterform at two sizes. Foundries knew this when type was cut in metal, lost it when photocomposition arrived, and variable fonts have finally made it adjustable again.
The golden canon, and the arithmetic under it
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.