This paper develops a theory for higher order continuity of ruled surf
aces constructed from ruled surface patches. Continuity is defined by
the equivalence of a minimal set of geometric properties for the ruled
surfaces and their evolutes using the dual are element as the paramet
er of the surface. The method used is a completely new approach to hig
her-order continuity of ruled surfaces. Also, contact continuity is co
mpared to continuity of the Frenet trihedrons and the curvatures. Thes
e continuity conditions can be used in design procedures for ruled sur
face segments in CAGD. (C) 1998 Elsevier Science B.V.