\begin{definition}[Definition:Reciprocal Proportion]
Let $P$ and $Q$ be geometric figures of the same type (that is, having the same number and configuration of sides).
Let $A$ and $B$ be sides of $P$, and let $C$ and $D$ be sides of $Q$, such that $A$ and $C$ are corresponding sides, and $B$ and $D$ also be corresponding sides.
Then $P$ and $Q$ have sides which are '''in reciprocal proportion''', or are '''reciprocally proportional''', if:
:$A : D = B : C$
where $A : D$ is the ratio of the lengths of $A$ and $D$.
\end{definition}