permutation

In probability and statistics, a permutation refers to the total number of ways a subset of a particular size can be chosen from a larger set, where the order of choice matters. The general formula is given by
    P(n,r) = n! / r!
where n! is n factorial, that is n × (n−1) × (n−2) × ... × 3 × 2 × 1
The term P(n,r) is often written Pⁿ_r.

For example, given four letters A, B, C, D, there are precisely twelve permutations of two items: AB, AC, AD, BA, BC, BD, CA, CB, CD, DA, DB, DC.
    C(4,2) = 4! / 2! = 4×3×2×1 / 2×1 = 24 / 2 = 12

Permutations are related to combinations, but the latter counts the number of unordered selections.