A sequence of type sizes produced by multiplying a base size by a fixed ratio, so every size on the page is mathematically related to every other.
Sizes come from a 1.25 scale on a 16px base: 16, 20, 25, 31, never from eyeballing.
The ratio is the typographic voice: 1.2 murmurs, 1.333 speaks, 1.618 is a poster. A scale is a constraint you set once so you never argue with yourself again. It’s also the cheapest way to make AI-generated layouts look governed, because models pick sizes by vibes unless you hand them the math. Break the scale only at the top, where display type and optical size demand their own rules.