How does one measure the “opening” between two half-planes in space? One introduces the dihedral angle and reduces it to a plane angle.
Definition — Dihedral angle
A dihedral angle is the portion of space bounded by two half-planes having a line in common, called the edge of the dihedron. To measure it one draws a plane to the edge: this cuts the two half-planes along two rays, and the angle thus identified is the normal section. Its magnitude is, by definition, the measure of the dihedral angle.
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Topics: Synthetic geometry in space
Concepts: Dihedral angle · Normal section
Skills: Synthetic geometry