  The coalescence of water drops on a substrate is studied experimentally. We
focus on the rapid growth of the bridge connecting the two drops, which very
quickly after contact ensues from a balance of surface tension and liquid
inertia. For drops with contact angles below $90^\circ$, we find that the
bridge grows with a self-similar dynamics that is characterized by a height
$h\sim t^{2/3}$. By contrast, the geometry of coalescence changes dramatically
for contact angles at $90^\circ$, for which we observe $h\sim t^{1/2}$, just as
for freely suspended spherical drops in the inertial regime. We present a
geometric model that quantitatively captures the transition from 2/3 to 1/2
exponent, and unifies the inertial coalescence of sessile drops and freely
suspended drops.
