We consider the two above observables A and B . Suppose there exists a complete set of kets { | ψ ψ n ⟩ ⟩ } whose every element is simultaneously an eigenket of A and B . Then we say that A and B are compatible. If we denote the eigenvalues of A and B corresponding to | ψ ψ n ⟩ ⟩ respectively by a n and b n , we can write