Pattern Library

A pattern library is a documented collection of the reusable interface pieces your site is built from, including buttons, cards, form fields, navigation, and other components, shown with examples and notes on how to use them.

It is a practical, working catalog. Where a design system captures the whole philosophy, the pattern library is the bucket of LEGO that everyone reaches into. It keeps designers and developers speaking the same language, so a card means the same thing in a mockup as it does in the code.