Ruled surface defines a surface by plotting rules between two different curves. The curves can be open or closed, planar or non-planar. For closed curves, the starting points of the curves must line up or the surface may twist in odd ways. See Twists in surfaces or solids with closed cross sections for more information. Ruled surfaces are useful for filling a region and blending between two curves and can be used with more than two curves to create a single surface where each section is the same as if each pair of curves were used to create a ruled surface.
Chained curves behave differently in a ruled surface than spline curves. If your surface does not look quite right in the Preview, select the Reparameterize curves check box and try the Preview again. Reparameterize curves analyzes the curves and may adjust the control points of the curves to yield a better surface result. Reparameterize affects both chained and spline curves, but the effect is stronger on chained curves.
Ruled surfaces are exact.