From Mathwold "A cubic spline is a spline constructed of piecewise third-order polynomials which pass through a set of m control points." Draw a 3 sided polygon, duplicate and you'll see that if you change the second vertex to a cubic vertex (or smooth cubic) the graphical representation doesn't mach the mathematical description... the curve doesn't pass trough the cubic bezier (zoom in). Now draw the line between the first and the third vertex, and another one from the middle of this last o