A type scale is a small set of sizes that agree with each other. Pick a base, pick a ratio, and let the steps fall where the maths puts them — then bend the top end by hand for display.
Base, ratio, exceptions
Most of a interface lives at one or two sizes. The scale exists for the moments that don't — the masthead, the caption, the number that has to be read across a room.
A scale is a promise the page keeps.
