In geometry, two polygons are said to be congruent if they are exact copies or exact mirror images of each other.

Triangles (three-sided polygons) are congruent if they follow any of the five following rules:

- SSS: All three sides are equal
- SAS: 2 sides and their included angle are equal
- ASA: A pair of angles and their included side are equal
- AAS: 2 corresponding angles and their non-included side are equal
- HL: The hypotenuse and 1 leg of a right triangle are equal

Another way to explain congruency is to say that if you made a paper cutout of the two polygons they would exactly fit when placed on top of each other, except in the case of mirror images, one must be turned over.

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