  The characteristic initial value problem has been implemented as a robust
computational algorithm (the PITT NULL CODE), with direct application to binary
black holes. The event horizon can be analyzed by characteristic techniques as
a stand-alone object using an analytic conformal model which gives new insight
into the intrinsic geometry of binary black holes. When applied to a
non-axisymmetric horizon, the model reveals substantially new features.
Colliding black holes generically go through a toroidal phase before they
become spherical. The conformal structure of the horizon supplies part of the
data for a simulation of the exterior space-time and calculation of the
post-merger waveforms from a binary black hole inspiral.
