  AM Her variables -- synchronised magnetic cataclysmic variables (CVs) --
exhibit a different period distribution from other CVs across the period gap.
We show that non-AM Her systems may infiltrate the longer-period end of the
period gap if they are metal-deficient, but that the position and width of the
gap in orbital period is otherwise insensitive to other binary parameters
(excepting the normalisation of the braking rate). In AM Her binaries, magnetic
braking is reduced as the wind from the secondary star may be trapped within
the magnetosphere of the white dwarf primary. This reduced braking fills the
period gap from its short-period end as the dipole magnetic moment of the white
dwarf increases. The consistency of these models with the observed distribution
of CVs, both AM Her and non-AM Her type, provides compelling evidence
supporting magnetic braking as the agent of angular momentum loss among
long-period CVs, and its disruption as the explanation of the 2 - 3 hour period
gap among nonmagnetic CVs.
