In this lesson, students review triangle congruence criteria and practice writing proofs that use triangle congruence theorems. Students begin by identifying corresponding parts in a pair of congruent triangles and practice naming the theorem that proves the triangles are congruent. Students continue that practice in the following activity with pairs of triangles that aren’t necessarily congruent. Finally, students use triangle congruence theorems to write a proof about two triangles formed by the diagonals of a quadrilateral. The proof can be written using any of the three triangle congruence theorems. Students who complete a proof early will be asked to try the proof using a different congruence theorem, which encourages them to make sense of the problem and persevere in solving it (MP1).