Modular scale

A series of type sizes derived from one ratio, each step a fixed multiple of the last. Sizes that are related, not merely chosen.

A modular scale replaces taste-by-the-pixel with a small system: pick a base size and a ratio, and the scale yields the sizes, caption, body, subhead, display, each a chord in the same key. Layouts built on a scale agree with themselves without anyone supervising the agreement.

The ratio is the voice: minor thirds step politely, perfect fourths stride, golden-ratio scales leap theatrically. The discipline is staying on the scale under pressure, when a headline wants to be just a bit bigger, the answer is the next step, not an improvised size. Exceptions are permitted; unsystematic exceptions multiply.