Letterform structure
The measurements underneath every hand: x-height ratios, counters, apertures, and the size at which writing stops being readable.
x-height: the one number every other letterform measurement is divided by
Letterforms are not measured in millimetres, they are measured in x-heights. The x-height is the distance between the baseline and the top of a lower-case x, and once it is fixed every other dimension in the alphabet becomes a ratio against it: ascenders, descenders, capitals, stroke weight, even the spacing between letters.
Counter and aperture: the two enclosed spaces that decide whether writing survives being small
The counter is the enclosed space inside a letter; the aperture is the size of the opening that lets ink and light into it. Both are invisible when you look at a letter as a shape and decisive when you look at a page that has to be read. This page is about the two numbers that govern them and the practical limit they impose on writing small.