The term congruent means two shapes have the same shape and size (but not necessarily the same orientation or location). For example, if the lengths of two lines are equal, the two lines themselves are congruent. Congruence is written using the symbol ≅. In figures, congruent parts are denoted with hash marks.

Shapes that are similar have the same shape but the not the same size, meaning their corresponding angles are the same but their lengths are not. For two shapes to be similar, the ratio of their corresponding sides must be a constant (called the scale factor). Similarity is described using the symbol ~.