Definition 2.23: If(A, <)is a poset and every elements of A are comparable, A is called a totally ordered or linearly ordered set, and s is called a total order or linear order. A totally ordered set is also called a chain The relation s on z is a total order. the relation on Z is not a total order. The relation c on the power of a set A is not a total orderDefinition 2.23：If (A, ≼) is a poset and every elements of A are comparable, A is called a totally ordered or linearly ordered set, and ≼ is called a total order or linear order. A totally ordered set is also called a chain The relation ≦ on Z is a total order. The relation | on Z is not a total order. The relation  on the power of a set A is not a total order