# Dihedral symmetry in three dimensions

 Polyhedral group, [n,3], (*n32) Involutional symmetryCs, (*)[ ] =  Cyclic symmetryCnv, (*nn)[n] =    Dihedral symmetryDnh, (*n22)[n,2] =      Tetrahedral symmetryTd, (*332)[3,3] =      Octahedral symmetryOh, (*432)[4,3] =      Icosahedral symmetryIh, (*532)[5,3] =     In geometry, dihedral symmetry in three dimensions is one of three infinite sequences of point groups in three dimensions which have a symmetry group that as abstract group is a dihedral group Dihn ( n  2 ).

## Types

There are 3 types of dihedral symmetry in three dimensions, each shown below in 3 notation: Schönflies notation, Coxeter notation, and orbifold notation.

Chiral
• Dn, [n,2]+, (22n) of order 2ndihedral symmetry or para-n-gonal group (abstract group Dihn)
Achiral
• Dnh, [n,2], (*22n) of order 4nprismatic symmetry or full ortho-n-gonal group (abstract group Dihn × Z2)
• Dnd (or Dnv), [2n,2+], (2*n) of order 4nantiprismatic symmetry or full gyro-n-gonal group (abstract group Dih2n)

For a given n, all three have n-fold rotational symmetry about one axis (rotation by an angle of 360°/n does not change the object), and 2-fold about a perpendicular axis, hence about n of those. For n = ∞ they correspond to three frieze groups. Schönflies notation is used, with Coxeter notation in brackets, and orbifold notation in parentheses. The term horizontal (h) is used with respect to a vertical axis of rotation.

In 2D the symmetry group Dn includes reflections in lines. When the 2D plane is embedded horizontally in a 3D space, such a reflection can either be viewed as the restriction to that plane of a reflection in a vertical plane, or as the restriction to the plane of a rotation about the reflection line, by 180°. In 3D the two operations are distinguished: the group Dn contains rotations only, not reflections. The other group is pyramidal symmetry Cnv of the same order.

With reflection symmetry with respect to a plane perpendicular to the n-fold rotation axis we have Dnh [n], (*22n).

Dnd (or Dnv), [2n,2+], (2*n) has vertical mirror planes between the horizontal rotation axes, not through them. As a result the vertical axis is a 2n-fold rotoreflection axis.

Dnh is the symmetry group for a regular n-sided prisms and also for a regular n-sided bipyramid. Dnd is the symmetry group for a regular n-sided antiprism, and also for a regular n-sided trapezohedron. Dn is the symmetry group of a partially rotated prism.

n = 1 is not included because the three symmetries are equal to other ones:

• D1 and C2: group of order 2 with a single 180° rotation
• D1h and C2v: group of order 4 with a reflection in a plane and a 180° rotation through a line in that plane
• D1d and C2h: group of order 4 with a reflection in a plane and a 180° rotation through a line perpendicular to that plane

For n = 2 there is not one main axes and two additional axes, but there are three equivalent ones.

• D2 [2,2]+, (222) of order 4 is one of the three symmetry group types with the Klein four-group as abstract group. It has three perpendicular 2-fold rotation axes. It is the symmetry group of a cuboid with an S written on two opposite faces, in the same orientation.
• D2h, [2,2], (*222) of order 8 is the symmetry group of a cuboid
• D2d, [4,2+], (2*2) of order 8 is the symmetry group of e.g.:
• a square cuboid with a diagonal drawn on one square face, and a perpendicular diagonal on the other one
• a regular tetrahedron scaled in the direction of a line connecting the midpoints of two opposite edges (D2d is a subgroup of Td, by scaling we reduce the symmetry).

## Subgroups D2h, [2,2], (*222) D4h, [4,2], (*224)

For Dnh, [n,2], (*22n), order 4n

• Cnh, [n+,2], (n*), order 2n
• Cnv, [n,1], (*nn), order 2n
• Dn, [n,2]+, (22n), order 2n

For Dnd, [2n,2+], (2*n), order 4n

• S2n, [2n+,2+], (n×), order 2n
• Cnv, [n+,2], (n*), order 2n
• Dn, [n,2]+, (22n), order 2n

Dnd is also subgroup of D2nh.

## Examples

D2h, [2,2], (*222)
Order 8
D2d, [4,2+], (2*2)
Order 8
D3h, [3,2], (*223)
Order 12  baseball seam paths Beach ball
(ignoring colors)

Dnh, [n], (*22n): prisms

D5h, , (*225): Pentagrammic prism Pentagrammic antiprism

D4d, [8,2+], (2*4): Snub square antiprism

D5d, [10,2+], (2*5): Pentagonal antiprism Pentagrammic crossed-antiprism pentagonal trapezohedron

D17d, [34,2+], (2*17): Heptadecagonal antiprism