Start here

The Concepts library is an evergreen reference for system-design ideas. Unlike the lessons — which are short, interactive, and built around questions — concepts are long-form articles you can read anytime, in any order, without starting a lesson.

How to use it

• Browse by category on the Concepts index.
• Each concept links back to the skills that practise it, under "Practice this", so you can read the theory and then drill it.
• Lessons may also link here from their results screen as a "Go deeper" read.

What's coming

We're filling this library with in-depth explanations sourced from the same references the lessons are built on. If a topic you want isn't here yet, it's probably on the way.