The Envelope deformation allows you to create an envelope around a drawing, then deform the entire drawing by manipulating the envelope's points and curves. They can be used to deform parts of a character with a fluid shape, such as hair, cloaks, shoulders, chins and so on. Envelope deformations can also be used to deform rigid parts of a characters so as to make them look like they are seen from different angles. This is commonly done to make character rigs rotate without swapping drawings, and using this technique makes it possible to animate a character rig going from one angle to another without having to swap drawings or character models.
Envelope deformations are very similar to Curve deformations, except that their last point connects to their first point to make a closed shape. Also, contrary to Curve deformations, all points move independently from one another, whereas the first point of a Curve deformation offsets all its other points.