  Detached, inspiraling and semi-detached, mass-transferring double white dwarf
(DWD) binary systems are both expected to be important sources for the proposed
space-based gravitational-wave detector, LISA. The mass-radius relationship of
individual white dwarf stars in combination with the constraints imposed by
Roche geometries permit us to identify population boundaries for DWD systems in
LISA's ``absolute'' amplitude-frequency diagram. With five key population
boundaries in place, we are able to identify four principal population
sub-domains, including one sub-domain that identifies where progenitors of Type
Ia supernovae will reside. Given one full year of uninterrupted operation, LISA
should be able to measure the rate at which the gravitational-wave frequency
$f$ and, hence, the orbital period is changing in the highest frequency
subpopulation of our Galaxy's DWD systems. We provide a formula by which the
distance to each DWD system in this subpopulation can be determined; in
addition, we show how the masses of the individual white dwarf stars in
mass-transferring systems may be calculated.
