Having the boundaries oriented in opposite directions causes the blended surface to twist in on itself. So that a mapping between a point on each boundary can be established. The starting point of both boundaries are usually inadequate as mapping points and using them can cause unwanted twists on the surface. Using Construction History When a blended surface is first created, it has associated construction history. If any of the surfaces on which the selected surface curves used to define the boundaries is transformed, the blended surface will be reconstructed.
Construction history is fragile.
If the parameterization of the surface curve changes, the construction history is removed. The parameterization can change with operations such as attaching, detaching, opening or closing, and reversing direction. This function is useful for building blended surfaces at the joints of a skeletal model. For example, a blend between surfaces that define the upper arm and the forearm of a human figure could be built. Animating either the upper arm or the forearm causes the blended surface to be rebuilt at each frame.