In modular arithmetic, when you get to the end of your finite list of numbers, you just loop back to the beginning, as if you were counting numbers around the face of a clock. For example, if you go ...

You may never have heard of modular arithmetic, but you use it every day without the slightest difficulty. In this system, numbers wrap around when they reach a certain size called the modulus; it is ...

The Modular Method of Mahjong Keeping track of the game is an exercise in the useful arithmetic of remainders. So is reading a clock.