A Relation denotes any kind of relationship between two objects in a [[Set]]. A [[Relation]] between two [[Set|Sets]] $X,Y$ can be viewed as a [[Set]] of [[Ordered Pairs]] $(x,y)$ such that $x\in X, y \in Y$. The [[Set]] of all possible [[Relation|Relations]] betwen two [[Set|Sets]] $X,Y$ is equal to $X \times Y$. >[!example] [[Function|Functions]] The most used form is a [[Function]], where a [[Function]] is a [[Relation]] between two [[Set|Sets]] that is right-unique and left-total (every input has *exactly* one output).