There seems to be neither rhyme nor reason for the naming of the states corresponding to the different values of l (s, p, d, f for l = 0, 1, 2, 3). This set of labels had its origin in the early work of experimental atomic spectroscopy. The letter s stood for sharp, p for principal, d for diffuse and f for fundamental in characterizing spectral lines. From the letter f onwards the naming of the orbitals is alphabetical l =  4,5,6; —g,h,i, . . . .