\begin{definition}[Definition:Euclid's Definitions - Book XI/4 - Plane at Right Angles to Plane]
{{EuclidSaid}}
:''A '''plane''' is '''at right angles to a plane''' when the straight lines drawn, in one of the planes, at right angles to the common section of the planes are at right angles to the remaining plane.''
{{EuclidDefRef|XI|4|Plane at Right Angles to Plane}}
\end{definition}