Ch 6.6 Two observables $\hat{A}$ and $\hat{B}$ that commute: $[\hat{A}, \hat{B}] = 0$. They share a complete set of simultaneous eigenstates, and measuring one does not disturb the value of the other. *See also:* Commutator, Incompatible observables, Complete set of commuting observables (CSCO).