Two distinct half-planes emanating from the same line l divide space (ℝ3) into two regions called
dihedral angles. The line l is the edge of the dihedral angle, and the bounding half-planes are its
sides.
A simple sketch of a dihedral angle with edge l.
The angle which the sides of the dihedral angle determine in a plane normal to the edge is the
normal section of the dihedral angle. Apparently, all such normal sections are equal. According to
the size of the normal section, the dihedral angle may be called acute, right, obtuse, straight, skew,
convex, or concave. Unlike the angle between two planes, a dihedral angle may be greater than
90∘.
If two planes intersect each other and one of the four dihedral angles formed is right, then the
others are right as well. In that case, we say that the planes are perpendicular to each
other.
