Definition:Right Angle/Perpendicular/Plane to Plane

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In the words of Euclid:

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.

(The Elements: Book $\text{XI}$: Definition $4$)


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.