Dupin's theorem

Dupin's theorem

[dyü′paz ‚thir·əm]
(mathematics)
The proposition that, given three families of mutually orthogonal surfaces, the line of intersection of any two surfaces of different families is a line of curvature for both the surfaces.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.