Triangles [Congruency in Triangles]
206. Same Shape, Same Size · The definition of congruent triangles
Corresponding sides and angles stay equal, so triangle DEF can always be slid to coincide exactly with ABC.
Two triangles are congruent (written ΔABC ≅ ΔDEF) when they have the same shape and size — every pair of corresponding sides is equal (AB = DE, BC = EF, CA = FD) and every pair of corresponding angles is equal. Congruent triangles can be superimposed, fitting exactly one over the other. Drag D to slide triangle DEF onto ABC — it covers it perfectly.