The transitions are divided into two classes, those which do not change parity change $\pi _{i}\pi _{f}=+1$ which go by the operators with $\pi _{O}=+1$: $$\pi _{i}\pi _{f}=+1 \; \mathrm{for} \; M1, E2, M3, E4 \dots,$$ and the ones which do change parity change $\pi _{i}\pi _{f}=-1$ which go by the operators with $\pi _{O}=-1$: $$\pi _{i}\pi _{f}=-1 \; \mathrm{for} \; E1, M2, E3, M4 \dots.$$