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.
If two figures are congruent, then their corresponding sides and angles are also congruent.
If two figures are similar, their corresponding sides are proportional and their corresponding angles are congruent.