I'm pretty comfortable with special relativity, and at least familiar with the principles of the general theory, but recently I've tried to learn SR using tensors. It is my first foray into this branch of mathematics. I understand they're handy because they represent invariant objects, but the learning is otherwise slow going. One basic question I can't seem to be able to find a direct answer to is this: What does it mean for something to be covariant or contravariant? In other words, what's the difference between the two? When I see equations in my books they seem to randomly choose which quantities are represented by each one. Could anyone give me some examples of each and tell me how they were decided to be what they are? Thanks.