Proposition: 6.15: Characterization of Congruent Triangles

(Proposition 15 from Book 6 of Euclid's “Elements”)

In equal triangles also having one angle equal to one (angle) the sides about the equal angles are reciprocally proportional. And those triangles having one angle equal to one angle for which the sides about the equal angles (are) reciprocally proportional are equal.


Modern Formulation

Two triangles are congruent if and only if they have one angle equal and the products1 of the side lengths of this angle are in both triangles equal.

$$\begin{array}{rclc} \bigtriangleup{ABC}\cong\bigtriangleup{ADE}&\Longleftrightarrow&(\angle{BAC}=\angle{DAE})&\wedge\\ &&(|\overline{CA}|\cdot|\overline{AB}|=|\overline{EA}|\cdot|\overline{AD}|). \end{array}$$

Proofs: 1

Proofs: 1 2

Thank you to the contributors under CC BY-SA 4.0!



Adapted from (subject to copyright, with kind permission)

  1. Fitzpatrick, Richard: Euclid's "Elements of Geometry"

Adapted from CC BY-SA 3.0 Sources:

  1. Prime.mover and others: "Pr∞fWiki",, 2016


  1. The product is equivalent to Euclid's "reciprocal proportion" $$\frac{|\overline{CA}|}{|\overline{AD}|}=\frac{|\overline{EA}|}{|\overline{AB}|},$$ which is not to be confused with the concept of reciprocal ratio