Euclidean geometry Plane geometry

Two triangles are said to be congruent if one can be exactly superimposed on the other by a rigid motion, and the congruence theorems specify the conditions under which this can occur. The first theorem illustrated in the diagram is the side-angle-side (SAS) theorem: If two sides and the included angle of one triangle are equal to two sides and the included angle of another triangle, the triangles are congruent. Following this, there are corresponding angle-side-angle (ASA) and side-side-side (SSS) theorems.

The first very useful theorem derived from the axioms is the basic symmetry property of isosceles triangles—i.e., that two sides of a triangle are equal if and only if the angles opposite them are equal. Euclid’s proof of this theorem was once called Pons Asinorum (“Bridge of Asses”), supposedly because mediocre students could not proceed across it to the farther reaches of geometry. (For an illustrated exposition of the proof, see Sidebar: The Bridge of Asses). The Bridge of Asses opens the way to various theorems on the congruence of triangles.

The parallel postulate is fundamental for the proof of the theorem that the sum of the angles of a triangle is always 180 degrees. A simple proof of this theorem, attributed to the Pythagoreans, is shown in the diagram.