  In his 1990 doctoral thesis, Todd Drumm showed that proper affine
deformations of free Fuchsian groups could be constructed as Schottky groups
using a new family of hypersurfaces called "crooked planes." The existence of
proper affine deformations of Fuchsian Schottky groups was demonstrated by
Margulis in the early 1980's, answering a question raised by Milnor in 1977.
This paper expounds Drumm's result, at least in the case of Fuchsian Schottky
groups (that is, when the group contains no parabolic elements).
