Five understandable doors.
- Electrical — DC/AC fundamentals, batteries, wiring, protection and field power.
- Mechanical — machines, force, motion, hydraulics and practical diagnosis.
- Water — pumps, pressure, flow and remote-property systems.
- Structures — loads, bracing, foundations and construction reasoning.
- Troubleshooting — observe, measure, isolate, test and prove the repair.
The deep lessons, calculators and field material can live underneath these doors without greeting every beginner with the entire catalogue.