Energy-level diagram of NH₃, where the rotational quantum number, J, is placed to the left of each level, the symmetry index, ϵ = + or −, is placed to the right of each level. The symmetry index is that defined by Rist et al. (1993), which might differ from common spectroscopic notation. The transitions are marked with colored arrows, where the direction corresponds to absorption. The gray arrow represents the 1810 GHz transition observed with SOFIA by Wyrowski et al. (2012).