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A symmetric relation is a type of binary relation. Formally, a binary relation R over a set X is symmetric if:

where the notation aRb means that (a, b) ∈ R.

An example is the relation "is equal to", because if a = b is true then b = a is also true. If R represents the converse of R, then R is symmetric if and only if R = R.

Symmetry, along with reflexivity and transitivity, are the three defining properties of an equivalence relation.