A dihedral angle is the union of any line and two half-planes that have the line as a common edge. Like a thin piece of paper that has one fold. It has a face which is the union one of the half-planes and the dihedral edge. The dihedral edge, subsequently, is one line: the common edge of the fold.
The notation for a dihedral angle is similar to that of an angle on flat plane. For example, A-BC-D, where A and D are each points on one of the faces, and BC represents the line which makes the dihedral edge. Most of the theorems and postulates that apply to the regular angles found in planes also apply to dihedral angles.