In mathematics, closure describes the case when the results of a mathematical operation are always defined. For example, in ordinary arithmetic, addition on real numbers has closure: whenever one adds two numbers, the answer is a number. The same is true of multiplication. Division does "not" have closure, because division by 0 is not defined. In the natural numbers, subtraction does not have closure, but in the integers, subtraction does have closure. Subtraction of two numbers can produce a negative number, which is not a natural number, but is an integer.