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Dihedron
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<p>A dihedron is a type of <a href="/facts/Polyhedron/UC0p9FTF">polyhedron</a>, made of two <a href="/facts/Polygon/yAq7RjPz">polygon</a> faces which share the same set of <i>n</i> <a href="/facts/Polyhedron/UC0p9FTF">edges</a>. In three-dimensional <a href="/facts/Euclidean_space/R2UbzmzM">Euclidean space</a>, it is <a href="/facts/Mathematical_degeneracy/BqhqlXPH">degenerate</a> if its faces are flat, while in three-dimensional <a href="/facts/Spherical_3-manifold/8zJWCqh9">spherical space</a>, a dihedron with flat faces can be thought of as a lens, an example of which is the fundamental domain of a <a href="/facts/Lens_space/5hQ8dgA5">lens space</a> L(<i>p</i>,<i>q</i>). Dihedra have also been called bihedra, flat polyhedra, or doubly covered polygons. </p><p>As a <a href="/facts/Spherical_tiling/jaglysFR">spherical tiling</a>, a dihedron can exist as nondegenerate form, with two <i>n</i>-sided faces covering the sphere, each face being a <a href="/facts/Sphere/jywYLNM2">hemisphere</a>, and vertices on a <a href="/facts/Great_circle/x4fq6eNm">great circle</a>. It is regular if the vertices are equally spaced. </p><p>The <a href="/facts/Dual_polyhedron/rkIHbmAf">dual</a> of an <i>n</i>-gonal dihedron is an <i>n</i>-gonal <a href="/facts/Hosohedron/t2dyV0bW">hosohedron</a>, where <i>n</i> <a href="/facts/Digon/vur5IQ5d">digon</a> faces share two vertices. </p>