Equality of triangles

Two triangles are called equal (Δ A1B1C1 = Δ A2B2C2) if their respective sides are equal and the corresponding angles are equal. A1B1 = A2B2, B1C1 = B2C2, C1A1 = C2A2 and ∠ B1A1C1 = ∠ B2A2C2, ∠ A1B1C1 = ∠ A2B2C2, ∠ B1C1A1 = ∠ B2C2A2.

Equal angles are usually graphically marked with one, two or three bows.

Axiom

Whatever the triangle, there is an equal triangle in a given location relative to this half-line.