\begin{definition}[Definition:Euclid's Definitions - Book I/19 - Rectilineal Figure]
{{EuclidSaid}}
:'''''Rectilineal figures''' are those which are contained by straight lines, '''trilateral''' figures being those contained by three, '''quadrilateral''' those contained by four, and '''multi-lateral''' those contained by more than four straight lines.''
{{EuclidDefRef|I|19|Rectilineal Figure}}
\end{definition}