For those interested in some details.

A mulit-span NURBS curve or surface without multi-knots has continuity between which is one degree less than rhw degree of the curve or surface. A degree 3 curve or surface without multi-knots has C2 continuity between spans. Knots are located between spans. If there is a multi-knot then the continuity at the multi-knot is reduced.

Notice that above i said C2 continuity, not G2 continuity. There is a difference. G2 continuity is curvature continuity. G continuity is geometric continuity and G2 continuity means the curvature is continuous. C continuity is more rigorous and means derivatives are continuous. C2 continuity means the second derivative is conitinuus. A C2 curve or surface is always G2 continuous but a G2 curve or surface may or may not be C2 continuous.

A curve or surface with mulit-knots will have reduced “C” continuity. The “G” continuity may or may not be reduced.

Why do commands such as Sweep2, DevSrf and DevLott sometimes produce surfaces with mulit-knots. The “simple” answer is because a single surface without multi-knots wihich exactly matches the input curves may not be mathematically possible because of differences in the parameterization of input curves…