In mathematics, modular arithmetic is a system of arithmetic for certain equivalence classes of integers, called congruence classes. Sometimes it is suggestively called 'clock arithmetic', where numbers 'wrap around' after they reach a certain value (the modulus). For example, when the modulus is 12, then any two numbers that leave the same remainder when divided by 12 are equivalent (or "congruent") to each other.

Wikimedia Commons has media related to Modular arithmetic.