# Trapezohedron Geometric shape where the faces are congruent and symmetrically staggered

An n-gonal trapezohedron, antidipyramid, antibipyramid, or deltohedron is the dual polyhedron of an n-gonal antiprism. The 2n faces of an n-trapezohedron are congruent and symmetrically staggered; they are called twisted kites. With a higher symmetry, its 2n faces are kites (also called trapezoids,[citation needed] or deltoids).

The n-gon part of the name does not refer to faces here, but to two arrangements of each n vertices around an axis of n-fold symmetry. The dual n-gonal antiprism has two actual n-gon faces.

An n-gonal trapezohedron can be dissected into two equal n-gonal pyramids and an n-gonal antiprism.

These figures, sometimes called deltohedra, must not be confused with deltahedra, whose faces are equilateral triangles.

A twisted trigonal trapezohedron (with six twisted trapezoidal faces) and a twisted tetragonal trapezohedron (with eight twisted trapezoidal faces) exist as crystals; in crystallography (describing the crystal habits of minerals), they are just called trigonal trapezohedron and tetragonal trapezohedron. They have no symmetry plane, and no symmetry center. The trigonal trapezohedron has one 3-fold symmetry axis, perpendicular to three 2-fold symmetry axes. The tetragonal trapezohedron has one 4-fold symmetry axis, perpendicular to four 2-fold symmetry axes.

Also in crystallography, the word trapezohedron is often used for the polyhedron with 24 trapezoidal faces properly known as a (deltoidal) icositetrahedron. Another polyhedron, with 12 trapezoidal faces, is known as a deltoid dodecahedron.

The symmetry group of an n-gonal trapezohedron is Dnd, of order 4n, except in the case of n = 3: a cube has the larger symmetry group Od of order 48 = 4×(4×3), which has four versions of D3d as subgroups.

The rotation group of an n-trapezohedron is Dn, of order 2n, except in the case of n = 3: a cube has the larger rotation group O of order 24 = 4×(2×3), which has four versions of D3 as subgroups.

One degree of freedom within symmetry from Dnd (order 4n) to Dn (order 2n) changes the congruent kites into congruent quadrilaterals with three edge lengths, called twisted kites, and the n-trapezohedron is called a twisted trapezohedron. (In the limit, one edge of each quadrilateral goes to zero length, and the n-trapezohedron becomes an n-bipyramid.)

If the kites surrounding the two peaks are not twisted but are of two different shapes, the n-trapezohedron can only have Cnv (cyclic with vertical mirrors) symmetry, order 2n, and is called an unequal or asymmetric trapezohedron. Its dual is an unequal n-antiprism, with the top and bottom polygons of different radii.

If the kites are twisted and are of two different shapes, the n-trapezohedron can only have Cn (cyclic) symmetry, order n, and is called an unequal twisted trapezohedron.

An n-trapezohedron has 2n quadrilateral faces, with 2n+2 vertices. Two apexes are on the polar axis, and the other vertices are in two regular n-gonal rings of vertices.

A face-transitive star p/q-trapezohedron is defined by a regular zig-zag skew star 2p/q-gon base, two symmetric apexes with no degree of freedom right above and right below the base, and kite faces connecting each pair of base adjacent edges to one apex.

Such a star p/q-trapezohedron is a self-intersecting, crossed, or non-convex form. It exists for any regular zig-zag skew star 2p/q-gon base; but if p/q < 3/2, then pq < q/2, so the dual star antiprism (of the star trapezohedron) cannot be uniform (i.e.: cannot have equal edge lengths); and if p/q = 3/2, then pq = q/2, so the dual star antiprism must be flat, thus degenerate, to be uniform.

A dodecagonal trapezohedron is a trapezohedron with 24 kites. It has 12-fold antiprismmic symmetry, order 48.