\begin{definition}[Definition:Proportion/Joint]
Two real variables $x$ and $y$ are '''jointly proportional''' to a third real variable $z$ {{iff}} the product of $x$ and $y$ is a constant multiple of $z$:
:$\forall x, y \in \R: x y \propto z \iff \exists k \in \R, k \ne 0: x y = k z$
\end{definition}