\begin{definition}[Definition:Right Angle/Perpendicular/Plane to Plane]
{{EuclidDefinition|book = XI|def = 4|name = Plane at Right Angles to Plane}}
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In the above diagram, the two planes have been constructed so as to make lines perpendicular to their common section perpendicular to each other.
Thus the two planes are perpendicular to each other.
\end{definition}