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 \( \pi _{O}=-1 \): $$ \pi _{i}\pi _{f}=-1 \; \mathrm{for} \; E1, M2, E3, M4 \dots. $$