thumb|This diagram illustrates the geometric principle of angle-angle-side triangle congruence: given triangle ABC and triangle A'B'C', triangle ABC is congruent with triangle A'B'C' if and only if: angle CAB is congruent with angle C'A'B', and angle ABC is congruent with angle A'B'C', and BC is congruent with B'C'. Note hatch marks are used here to show angle and side equalities.
thumb|this diagram illustrates the geometric principle of angle-angle-side triangle congruence given triangle abc and triangle a'b'c' triangle abc is congruent with triangle a'b'c' if and only if angle cab is congruent with angle c'a'b' and angle abc is congruent with angle a'b'c' and bc is congruent with b'c' note hatch marks are used here to show angle and side equalities