  Measurements of black-hole spins from gravitational-wave observations of
black-hole binaries with ground-based detectors are expected to be hampered by
partial degeneracies in the gravitational-wave phasing: between the two
component spins, and between the spins and the binary's mass ratio, at least
for signals that are dominated by the binary's inspiral. Through the merger and
ringdown, however, a different set of degeneracies apply. This suggests the
possibility that, if the inspiral, merger and ringdown are all within the
sensitive frequency band of a detector, we may be able to break these
degeneracies and more accurately measure both spins. In this work we
investigate our ability to measure individual spins for non-precessing
binaries, for a range of configurations and signal strengths, and conclude that
in general the spin of the larger black hole will be measurable (at best) with
observations from Advanced LIGO and Virgo. This implies that in many
applications waveform models parameterized by only one \emph{effective spin}
will be sufficient. Our work does not consider precessing binaries or
sub-dominant harmonics, although we provide some arguments why we expect that
these will not qualitatively change our conclusions.
