  An edge tessellation is a tiling of the plane generated by reflecting a
polygon in its edges. We prove that a polygon generating an edge tessellation
is one the following eight types: a rectangle; an equilateral, 60-right,
isosceles right, or 120-isosceles triangle; a 120-rhombus; a 60-90-120 kite; or
a regular hexagon. A stamp folding puzzle is a paper folding problem
constrained to the perforations on a sheet of postage stamps. We establish the
following conjecture due to G. Frederickson: "Although triangular stamps have
come in a variety of different triangular shapes, only three shapes seem
suitable for [stamp] folding puzzles: equilateral, isosceles right triangles,
and 60-right triangles."
