The number of selections of n objects taken r at a time (that is, the number of ways of selecting r objects out of n) is denoted by ⁿC_r and is equal to

(See also binomial coefficient.) For example, from four objects A, B, C, and D, there are six ways of selecting two: AB, AC, AD, BC, BD, CD. The property that

can be directly seen in Pascal’s triangle. Compare permutation.