Bipyramid

In geometry, a bipyramid, dipyramid, or double pyramid is a polyhedron formed by fusing two pyramids together base-to-base. The polygonal base of each pyramid must therefore be the same, and unless otherwise specified the base vertices are usually coplanar and a bipyramid is usually symmetric, meaning the two pyramids are mirror images across their common base plane. When each apex (pl. apices, the off-base vertices) of the bipyramid is on a line perpendicular to the base and passing through its center, it is a right bipyramid;^[a] otherwise it is oblique. When the base is a regular polygon, the bipyramid is also called regular.
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