  The quantity $T_0$, the cosmic microwave background (CMB) monopole, is an
often neglected seventh parameter of the standard cosmological model. As well
as its variation affecting the physics of the CMB, the measurement of $T_0$ is
also used to calibrate the anisotropies, via the orbital dipole. We point out
that it is easy to misestimate the effect of $T_0$ because the CMB anisotropies
are conventionally provided in temperature units. In fact the anisotropies are
most naturally described as dimensionless and we argue for restoring the
convention of working with $\Delta T/T$ rather than $\Delta T$. As a free
cosmological parameter, $T_0$ most naturally only impacts the CMB power spectra
through late-time effects. Thus if we ignore the COBE-FIRAS measurement,
current CMB data only weakly constrain $T_0$. Even ideal future CMB data can at
best provide a percent-level constraint on $T_0$, although adding large-scale
structure data will lead to further improvement. The FIRAS measurement is so
precise that its uncertainty negligibly effects most, but not all, cosmological
parameter inferences for current CMB experiments. However, if we eventually
want to extract all available information from CMB power spectra measured to
multipoles $\ell\simeq5000$, then we will need a better determination of $T_0$
than is currently available.
