1×21

1×21 is the product of the integers 1 and 21 in arithmetic. The numerical result is 21. It illustrates the multiplicative identity: for any integer n, 1×n = n, and n×1 = n. Therefore 1×21 equals 21. Because multiplication is commutative, changing the order gives the same result: 21×1 = 21.

In a rectangular-array interpretation, 1×21 represents one row of 21 units, a visualization of how multiplication counts objects arranged in a grid. In base-n numeral systems, the symbol 21 represents the value 2·n + 1; multiplying by 1 leaves that value unchanged, so the product remains the same number in that base.

In practical arithmetic, 1×21 is often used to teach the identity property and to contrast with products involving 0 or larger multipliers, where the results differ more markedly. In algebra and higher mathematics, 1 is called the multiplicative identity because multiplying by it leaves elements unchanged; this property extends to broader algebraic structures such as rings and fields and underpins many theoretical concepts. See also multiplicative identity, basic arithmetic, and number representation.