x_{1} is a binary relationship which is symmetric (under exchange of arguments/terms) in space/under conditions/on set x_{2}.

Denote this binary relation x_{1} between elements y, z (in that order) by "y R z" (in that order); then, for any elements a, b in the set of consideration (possibly x_{2}), if a R b, then b R a. See also: kinra, takni.