Water energy level diagram for the lower portion of the vibrational ground state. As indicated, para levels are shown to the left and ortho levels are shown to the right. The state energy is given in Kelvin and is shown as a function of quantum number K_c. Near each level, the quantum numbers are listed. The 400 GHz transitions are shown as red lines, connecting upper and lower levels, with the frequency in gigahertz indicated. Some other relevant masing or thermal transitions are also included.