\begin{definition}[Definition:Polygon/Opposite]
When a polygon has an even number of sides, each side has an '''opposite side''', and each vertex likewise has an '''opposite vertex'''.
When a polygon has an odd number of sides, each side has an '''opposite vertex'''.
The '''opposite side''' (or '''opposite vertex''') to a given side (or '''vertex''') is that side (or vertex) which has the same number of sides between it and the side (or vertex) in question.
\end{definition}