J NURBS

This page has a perhaps more entertaining way to discuss surface continuity, but it also covers higher orders of continuity – G3 and G4.

https://www.augi.com/articles/detail/an-introduction-to-surface-continuity

If you’re curious to learn more about NURBS, I’m sharing a couple of links that go more in depth on how this geometry works.

This article links to a somewhat comprehensive definition of NURBS
http://www.rhino3d.com/nurbs

This article links to two other that are more deep in their description, with equations and formulas that are quite complex:
http://www.mactech.com/articles/develop/issue_25/schneider.html
http://web.cs.wpi.edu/~matt/courses/cs563/talks/nurbs.html

The Wikipedia page for NURBS is also a useful reference:
https://en.wikipedia.org/wiki/Non-uniform_rational_B-spline