Ch 6.3 For two operators $\hat{A}$ and $\hat{B}$: $[\hat{A}, \hat{B}] = \hat{A}\hat{B} - \hat{B}\hat{A}$. If $[\hat{A}, \hat{B}] = 0$, the operators commute and can be simultaneously diagonalized (compatible observables). The canonical commutation relation is $[\hat{x}, \hat{p}] = i\hbar$. *See also