T-Splines are truly a proper superset of NURBS [...]. Given two sets A and B, A is said to be a proper superset of B if every element of B is contained in A, and there exists at least one element in A that is not in B. By this basic definition, it is clear that the set of all T-Splines is a proper superset of the set of all NURBS, because every NURBS is a special case of a T-Spline, but not all T-Splines are NURBS. [Tom Sederberg, T-Splines, Inc.]