The rudiments of a subject are the first, simplest pieces you learn — the bits you need before anything else will make sense. If you are learning the piano, the rudiments are things like the names of the notes and how to sit at the keys. People also speak of the rudiments of cooking, of grammar, of a sport. The word is a little bookish; a teacher or a textbook is more likely to use it than a teenager on the bus. It almost always comes in the plural: we say the rudiments of chemistry, not a rudiment. Though actually, it is not a rude word, and it has nothing to do with being impolite. Long ago both ideas sat near raw and not yet trained. A close adjective, rudimentary, means very basic, not developed yet — a rudimentary shelter is only just a shelter.