Euclid's Elements Book I, Proposition 5: (Pons Asinorum)
In isosceles triangles the angles at the base are equal to one another, and, if the equal straight lines are produced further, then the angles under the base will be equal to one another.
Let ABC be an isosceles triangle having the side AB equal to the side BC, and let the straight lines BD, CE be produced further in a straight line with AB and
Pons Asinorum (Latin for "bridge of asses")
Pons asinorum is the name given to Euclid's
fifth proposition in Book 1 of his Elements of geometry because this
proposition is the first real test in the Elements of the intelligence
of the reader and as a bridge to the harder propositions that follow.
The Elements: Books I-XIII