Reader guide
How to read prerequisites
Every lesson declares the external topics you should know before starting, on a single 1–5 scale. Calibrate once and you can trust the callouts across the site.
The scale
- 1/5Middle school / pre-algebra
- Comfortable with arithmetic, fractions, powers of two
- Can follow a simple word problem
- 2/5High school level
- Algebra (solving for x, exponents, factoring)
- Geometry and basic trigonometry
- Intro physics — kinematics, circuits
- 3/5Early undergraduate
- Calculus I–II — set up and evaluate integrals in x, differentiate standard functions, apply u-substitution
- Intro programming in any language
- Basic linear algebra — matrices, dot products
- 4/5Upper-division undergraduate
- Junior/senior coursework in the major
- Signals & systems, discrete math, digital logic at a mature level
- Comfortable reading research-lite technical writing
- 5/5Graduating senior / entry-level professional
- Could step into a full-time role tomorrow
- Fluency with the topic under time pressure
Internal prerequisites
When a lesson says you should read another lesson first, there's no rating — the linked lesson speaks for itself, including its own external prereqs.
Badges on lesson cards
Cards show the highest external prereq rating on the lesson:
- No prereqsNo external prereqs declared
- Prereqs: 2/5Highest external prereq is level 1 or 2
- Prereqs: 3/5Highest external prereq is level 3
- Prereqs: 5/5Highest external prereq is level 4 or 5