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One way to describe congruent triangles is that two triangles are congruent if one fits on top of the other exactly without any change. The triangles only match if their corresponding parts match such as the sides and angles. In fact, it is necessary that there is a onetoone correspondence in equality or congruence between each part of one triangle to each part of the other for the two triangles to be congruent. The symbol used for representing a triangle is . The two direction arrow, ↔, is used to signify correspondence. The statement, ABC ↔ DEF means that the triangle with vertices ABC corresponds in all its parts with triangle DEF. There are six pairs of corresponding parts with three pairs being corresponding sides and three pairs corresponding angles. One definition for congruent triangles can be formally defined as follows. Given two triangles ABC and DEF, the correspondence of parts can be described as:
If all these equalities hold for the two triangles in question then the two triangles are congruent. The converse is also true if two triangles are congruent then the measures of their corresponding parts are equal. Direct CongruenceWhen trying to determine if two triangles are congruent or not it is also possible to classify them according to two types of congruence: direct or inverse. With direct congruence, the congruent triangles have the same orientation which means that if one is rotated and moved over and on top of the other one then they would fit exactly without flipping. Inverse CongruenceIf they have inverse congruence then one would have to be flipped over as well as possibly having to move and rotate it. Since inversely congruent triangles can still have corresponding parts, they satisfy the definition for congruent triangles. Another common way of describing congruent triangles is with the statement: Corresponding parts of congruent triangles are congruent. This statement is used often in geometry proofs involving congruent triangles and it is often abbreviated as: CPCTC. Congruence PostulatesIt is often possible to determine if two triangles are congruent by determining if three of the corresponding parts are congruent instead of all six. There are three basic postulates that relate three corresponding parts of one triangle to another: An angle or side is said to be included if it is between two of the same type of part. A side is included by the angles at its endpoints and an angle is included by the sides of the triangle that also form the sides of the angle. It is also possible to determine congruency if the two angles are given along with a side that is not included. This is sometimes stated as the following theorem: This can also be done by using the sum of angle measures of the triangle which is 180°. When the third angle is found, the ASA postulate can be used. There is another three part combination of the triangle called: SSA Side Side AngleThis involves two sides of a triangle with a nonincluded angle. With this type of combination, however, it is not possible to determine if two triangles are congruent. This is because the included angle between the two sides can have different measures. 
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