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An example of congruence. The two figures on the left are congruent, while the third is similar to them. The last figure is neither. Note that congruences alter some properties, such as location and orientation, but leave others unchanged, like distances and angles. The latter sort of properties are called invariants and studying them is the essence of geometry.

In geometry, two sets are called congruent if one can be transformed into the other by an isometry, i.e., a combination of translations, rotations and reflections. Less formally, two sets are congruent if they have the same shape and size, but are in different positions (for instance one may be rotated, flipped, or simply placed somewhere else).

In a Euclidean system, congruence is fundamental; it is the counterpart of an equals sign in numerical analysis. In analytic geometry, congruence may be defined intuitively thus: two mappings of figures onto one Cartesian coordinate system are congruent if and only if, for any two points in the first mapping, the Euclidean distance between them is equal to the Euclidean distance between the corresponding points in the second mapping.

A more formal definition: two subsets A and B of Euclidean space Rⁿ are called congruent if there exists an isometry f : Rⁿ → Rⁿ (an element of the Euclidean group E(n)) with f(A) = B. Congruence is an equivalence relation.

Two triangles are congruent if their corresponding sides and angles are equal. Usually it is sufficient to establish the equality of three corresponding parts and use one of the following results to conclude the congruence of the two triangles.

The shape of a triangle is determined up to congruence by specifying two sides and the angle between them (SAS), two angles and the side between them (ASA) or two angles and an adjacent side (AAS). Specifying two sides and an adjacent angle (SSA), however, usually yields two distinct possible triangles.

SAS (Side-Angle-Side): Two triangles are congruent if a pair of corresponding sides and the included angle are equal.

SSS (Side-Side-Side): Two triangles are congruent if their corresponding sides are equal.

ASA (Angle-Side-Angle): Two triangles are congruent if a pair of corresponding angles and the included side are equal. The ASA Postulate was contributed by Thales of Miletus (Greek).

In most system of axioms, the three criteria — SAS, SSS and ASA — are established as theorems. In the School Mathematics Study Group system SAS is taken as one (#15) of 22 postulates.

AAS (Angle-Angle-Side): Two triangles are congruent if a pair of corresponding angles and a not-included side are equal, since the 3rd angle would have to be equal, and therefore the side would be included.

The SSA (Side-Side-Angle) or ASS (Angle-Side-Side) condition does not guarantee congruence, because it is possible to have two incongruent triangles that satisfy the SSA conditions (two equal corresponding sides and an equal non-included angle). This is known as the ambiguous case. Specifically, SSA fails when the angle is acute, and the side opposite to the angle is shorter than the adjacent side, and the opposite side is longer than the adjacent side times the sine of the angle. In all other cases, SSA is valid.

Thus, the SSA condition does prove congruence when the angle is a right angle. This is known as the HL (Hypotenuse-Leg) condition, or the RHS (Right Angle-Hypotenuse-Side) condition. This is true because the hypotenuse of a right triangle is always longer than either leg.

The SSA condition is also valid if the angle is obtuse; or if the opposite side equals the adjacent side times the sine of the angle (in which case it is a right triangle). (For comparison notice that the opposite side cannot be smaller than the adjacent side times the sine of the angle as the triangle will not "close".)

AAA (Angle-Angle-Angle) says nothing about the size of the two triangles and hence shows only similarity and not congruence. In hyperbolic geometry though, this is sufficient for congruence.

• Euclidean plane isometry
• CPCTC

• The SSS
• The SSA
• Interactive animations demonstrating Congruent angles , Congruent line segments , Congruent triangles ,

Congruent polygons