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Two distinct half-planes, emanating from a same line  , divide the space (
 ) into two regions called dihedral angles. The line  is the edge of the dihedral angle and the bounding half-planes are its sides.
The angle, which the sides of a dihedral planes separate from a normal plane of the edge, is the normal section of the dihedral angle. Apparently, all normal sections are equal. According to the size of the normal section, the dihedral angle may be called acute, right, obtuse, straight, skew, convex and concave. Unlike the angle between two planes, a dihedral angle may be over 90 degrees.
If two planes intersect each other and if one of the four dihedral angles formed is right, then also the others are right. Then we say that the planes are perpendicular to each other.
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