Perpendicular Planes

Two intersecting planes are called perpendicular planes when they form a right dihedral angle (90°).

In other words, the angle between two perpendicular planes is 90° (a right angle).

They are also known as normal planes or orthogonal planes.

Perpendicular planes are distinct from oblique planes. Planes are considered oblique when they intersect but do not form a right dihedral angle, meaning their dihedral angle is not 90°.

When two planes form a right dihedral angle, they actually form three additional right dihedral angles because opposite dihedrals are congruent.

Thus, it can be said that two perpendicular planes divide three-dimensional space into four right dihedrals, each with a dihedral angle of 90°.

And so on.