  W UMa type binaries have two defining characteristics. These are (i) the
effective temperatures of both components are very similar, and (ii) the
secondary (currently less massive) component is overluminous for its current
mass. We consider the latter to be an indication of its mass before the
mass-transfer event. For these stars we define a mass difference ($\delta M$)
between the mass determined from its luminosity and the present mass determined
from fitting the binary orbit. We compare the observed values of the mass
difference to stellar models with mass loss. The range of initial secondary
masses that we find for observed W UMa type binaries is 1.3-2.6 M$_{\odot}$. We
discover that the A- and the W-subtype contact binaries have different ranges
of initial secondary masses. Binary systems with an initial mass higher than
$1.8 \pm 0.1$ M$_{\odot}$ become A-subtype while systems with initial masses
lower than this become W-subtype. Only 6 per cent of systems violate this
behavior. We also obtain the initial masses of the primaries using the
following constraint for the reciprocal of initial mass ratio: $0 < 1/q_i < 1$.
The range of initial masses we find for the primaries is 0.2-1.5 M$_{\odot}$,
except for two systems. Finally in comparing our models to observed systems we
find evidence that the mass transfer process is not conservative. We find that
only 34 per cent of the mass from the secondary is transferred to the primary.
The remainder is lost from the system.
