  The mass of a black hole is the sum of the irreducible mass and the mass
associated with the rotational or spin energy of the black hole. The
contribution of spin energy (divided by $c^2$) to the total black hole mass is
studied here for four samples of sources including 576 LINERs, 100 classical
double radio sources, 80 relatively local AGN, and 102 measurements of four
stellar mass X-ray binary systems. The spin mass-energy of a black hole may be
extracted causing the mass of the black hole to decrease. The ratio of spin
mass-energy to black hole mass ranges from about ten to thirty percent for the
sources studied here, where the maximum possible value of this quantity is
close to thirty percent. Typical fractions of the black hole mass available for
extraction for the samples studied are about $0.2 \pm 0.1$. The spin energy of
black holes represents a major reservoir that when tapped may impact the
immediate and extended black hole environment, decrease the black hole mass,
and perhaps modify relationships between black hole mass and galaxy properties.
These results are consistent with expectations based on other observations and
numerical simulations.
