Cauchy horizon end points and differentiability

Cauchy horizons are shown to be differentiable at end points where only a single null generator leaves the horizon. A Cauchy horizon fails to have any null generator end points on a given open subset iff it is differentiable on the open subset and also iff the horizon is (at least) of class C1 on the open subset. Given the null convergence condition, a compact horizon which is of class C2 almost everywhere has no end points and is (at least) of class C1 at all points.