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dihedral angle[dī′hē·drəl ‚aŋ·gəl]
a figure formed in space by two half-planes emanating from a single straight line and also the portion of space bounded by these half-planes (see Figure 1).
The half-planes are called the faces of the dihedral angle and their common straight line the edge. The dihedral angle is measured by the linear angle, that is, the angle α between two perpendiculars to the edge that emanate from one point and that lie in the different faces or, in other words, the angle formed by the intersection of the dihedral angle and a plane perpendicular to the edge.