Consider two figures. One figure is the image of the other under a transformation. The pairs formed by a part of the preimage — a side, angle, or vertex — and the image of that part are called corresponding parts. For example, in the following applet, A and P are corresponding vertices. By clicking on each part of ABCD, the corresponding part on PQRS will be highlighted. Note that the figures and their corresponding parts can either be congruent or similar.

Congruent or Similar?	Relationship
Congruent	The corresponding sides and angles of the figures are congruent.
Similar	The corresponding sides of the figures are proportional. The corresponding angles of the figures are also congruent.