Energy level diagram showing the doubling of the rigid rotor levels (identified by N_KaKc) for K_a = 0,1 into tunnelling sublevels labelled with the symmetry species of the complete internal wavefunction of CH₂OH in the C_2v(M) group. The statistical weights arising from the permutation of the two identical methylenic hydrogen nuclei are indicated between parentheses. Levels with a weight of 3 correspond to ortho-CH₂OH and those with a weight of 1 correspond to the para isomer of the radical. Examples of a-type transitions (not observed) are depicted as black, dashed lines, while representatives of b-type (observed) Q- and R-branch transitions are displayed in green and red, respectively. For clarity the tunnelling splittings (Δ) are 800 times the actual ones, and the fine- and hyperfine-structure splittings are not illustrated.