I'd like to comment on the previous 2 replies regarding G2 continuity...First...matching the rate of change of curvature is G3 continuity, NOT G2...a G2 requirement means the curvature value at the boundary are equal, not the rate of change is equal. Second, please do not assume the number of CV's controls continuity...If you create 2 flat degree 1 X 1 Z oriented planar surfaces with a common boundary, those 2 surface will be curvature continuous. In your scenario they could not be because there are not enough CV's. Next, the shape of geometry determines if it is curvature continuous, not a specific number of CV's. No software checks the number of CV's to determine continuity. NURBS modelers can create non-planar surfaces that do not have enough CV's to fulfill your 1st=pos 2nd=tan 3rd=curv concept that are curvature continuous. I would just like to clarify that fundamentally shape determines curvature continuity, not strictly a number of CV's...I will concede that users in most cases need 3 CV's to achieve curvature continuity, but that is not a requirement.