About

What a subject is

A book's worth of material rather than an introduction: enough that finishing one leaves you able to do something you could not do before. Each lesson teaches a single idea and leads into the next, so a subject can be read straight through. Some subjects continue from another and say so, and those sit together on the index.

Reading it

Every page is free, with no account, no paywall and no adverts. Text is quick to skim and easy to search, it prints, and it travels on a weak connection. Lessons carry the equations, diagrams and worked examples that real teaching needs, all rendered on the server, so a lesson arrives ready to read rather than assembling itself in your browser.

Testing yourself

Each lesson ends with a quiz you mark yourself: answer a question and it tells you at once, as many times as you like, because the point of it is to find out what did not stick. Each subject also has a final test, which is different. It is marked when you submit it rather than as you go, the score is kept on your profile, and it is sat once. That is the only thing an account is for.

Who writes it

The lessons are written by hand and kept as plain text files, which is what makes a whole subject something one person can hold in their head and revise as a whole. Nothing is edited through the site.

Languages

Each language has its own site and its own subjects, rather than translations of one original, so a course can be written for the people who will read it.