A Neusis construction is a geometric construction that breaks the strict rules of
classical Greek straightedge-and-compass construction (see constructible) by
allowing a marked ruler to be slid into different positions. Neusis construction
makes possible duplicating the
cube and trisecting an angle.
John Conway and Richard Guy have also shown how Neusis constructions, based on angle trisection, can
be used to draw regular polygons with 7, 9, and 13 sides.