| Layer | Domain | What It Provides |
|---|---|---|
| 7 | Product | Why to build; what users need |
| 6 | Code | How to implement correctly |
| 5 | Architecture | How components fit together |
| 4 | Infrastructure | What runs beneath the code |
| 3 | Data Structures | How data is organised for access |
| 2 | Algorithms | How problems are solved efficiently |
| 1 | Mathematics | The foundation; correctness proofs, complexity |
Use: Identify which layer a problem lives at. Debug downward — if the architecture is wrong, no amount of code quality fixes it. If the algorithm is wrong, no amount of infrastructure fixes it.
These cards are designed to be printed at A4 and taped near your monitor, or photographed and kept on your phone for pre-meeting review. The value is not in the cards themselves — it is in the retrievability they represent. When you can scan any card in 20 seconds and have it trigger full recall, you have reached the goal of this book.