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).

So, two perpendicular planes divide space into four right dihedral regions.

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.