  Greisen & Calabretta describe a generalized method for specifying the
coordinates of FITS data samples. Following that general method, Calabretta &
Greisen describe detailed conventions for defining celestial coordinates as
they are projected onto a two-dimensional plane. The present paper extends the
discussion to the spectral coordinates of wavelength, frequency, and velocity.
World coordinate functions are defined for spectral axes sampled linearly in
wavelength, frequency, or velocity, linearly in the logarithm of wavelength or
frequency, as projected by ideal dispersing elements, and as specified by a
lookup table.
