In geometry, two figures are congruent if they have the same shape and size. More formally, two sets of points are called congruent if, and only if, one can be transformed into the other by an isometry, i.e., a combination of translations, rotations and reflections.
The related concept of similarity permits a change in size.
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Definition of congruence in analytic geometry
In a Euclidean system, congruence is fundamental; it is the counterpart of equality for numbers. 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 R^{n} are called congruent if there exists an isometry f : R^{n} → R^{n} (an element of the Euclidean group E(n)) with f(A) = B. Congruence is an equivalence relation.
Congruence of triangles
Two triangles are congruent if their corresponding sides are equal in length and their corresponding angles are equal in size.
If triangle ABC is congruent to triangle DEF, the relationship can be written mathematically as:
In many cases it is sufficient to establish the equality of three corresponding parts and use one of the following results to deduce the congruence of the two triangles.
Determining congruence
Sufficient evidence for congruence between two triangles in Euclidean space can be shown through the following comparisons:
 SAS (SideAngleSide): If two pairs of sides of two triangles are equal in length, and the included angles are equal in measurement, then the triangles are congruent.
 SSS (SideSideSide): If three pairs of sides of two triangles are equal in length, then the triangles are congruent.
 ASA (AngleSideAngle): If two pairs of angles of two triangles are equal in measurement, and the included sides are equal in length, then the triangles are congruent.
The ASA Postulate was contributed by Thales of Miletus (Greek). In most systems 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 (AngleAngleSide): If two pairs of angles of two triangles are equal in measurement, and a pair of corresponding nonincluded sides are equal in length, then the triangles are congruent. (In British usage, ASA and AAS are usually combined into a single condition AAcorrS  any two angles and a corresponding side.)^{[1]}
 RHS (RightangleHypotenuseSide): If two rightangled triangles have their hypotenuses equal in length, and a pair of shorter sides are equal in length, then the triangles are congruent.
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