*X*(*u*,*v*) =*c*(*u*) +*v**z*(*t*)

Less formally, it is the surface swept out by moving a straight line
along the curve *c* so that it points in the direction
*z*(*t*) at each time *t*.

For example, the saddle surfaces *z* = *x y* and
*z* = *x*^2 - *y*^2 are both ruled surfaces, as is the
hyperboloid of one sheet, the helicoid, a cylinder and a cone.

* 8/12/94 dpvc@geom.umn.edu -- *

*The Geometry Center*