Definition:Comparable Elements

Definition
Let $\RR$ be a relation.

Two elements $x \in \Dom \RR$, $y \in \Img \RR$ such that $x \ne y$ are comparable if either:
 * $x \mathrel \RR y$

or:
 * $y \mathrel \RR x$

Also see
If $x$ and $y$ are not comparable then they are non-comparable.