One of the more familiar groups is the set of integers Z = { … … , − − 4 , − − 3 , − − 2 , − − 1 , 0 , 1 , 2 , 3 , 4 , … … } together with addition. For any two integers a and ⁠ b ⁠, the sum a + b is also an integer; this closure property says that + is a binary operation on ⁠ Z ⁠. The following properties of integer addition serve as a model for the group axioms in the definition below.