  CMB spectral distortions are induced by Compton collisions with electrons. We
review the various schemes to characterize the anisotropic CMB with a
non-Planckian spectrum. We advocate using logarithmically averaged temperature
moments as the preferred language to describe these spectral distortions, both
for theoretical modeling and observations. Numerical modeling is simpler, the
moments are frame-independent, and in terms of scattering the mode truncation
is exact.
