To control more complex or improbable shape changes, you can use shape hints. Shape hints identify points that should correspond in starting and ending shapes. For example, if you are tweening a drawing of a face as it changes expression, you can use a shape hint to mark each eye. Then, instead of the face becoming an amorphous tangle while the shape change takes place, each eye remains recognizable and changes separately during the shift.
Shape hints contain letters (a through z) for identifying which points correspond in the starting and ending shape. You can use up to 26 shape hints.
Shape hints are yellow in a starting keyframe, green in an ending keyframe, and red when not on a curve.
For best results when tweening shapes, follow these guidelines:
In complex shape tweening, create intermediate shapes and tween them instead of just defining a starting and ending shape.
Make sure that shape hints are logical. For example, if you're using three shape hints for a triangle, they must be in the same order on the original triangle and on the triangle to be tweened. The order cannot be abc in the first keyframe and acb in the second.
Shape hints work best if you place them in counterclockwise order beginning at the top left corner of the shape.
To use shape hints: