The longitudinal optical modes of a Fabry-Perot resonator are investigated. We consider (1) the three- and one-dimensional spectral mode density in free space and in standing-wave resonators, (2) the infinite sum of mode profiles, resulting in the Airy distribution, (3) comparison between a two-mirror resonator and an infinite periodic lensguide, and (4) comparison between a two-mirror resonator and a ring resonator. It is consistently deduced that the two counter-propagating waves with wave vectors ±|kq| at the same resonance frequency νq and polarization constitute independent optical modes with mode indices ±|q|.