Dihedral symmetry in three dimensions

Point groups in three dimensions

Involutional symmetry
Cs, (*)
[ ] =

Cyclic symmetry
Cnv, (*nn)
[n] =

Dihedral symmetry
Dnh, (*n22)
[n,2] =
Polyhedral group, [n,3], (*n32)

Tetrahedral symmetry
Td, (*332)
[3,3] =

Octahedral symmetry
Oh, (*432)
[4,3] =

Icosahedral symmetry
Ih, (*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 ).


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


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:

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


D2h, [2,2], (*222)

D4h, [4,2], (*224)

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

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

Dnd is also subgroup of D2nh.


D2h, [2,2], (*222)
Order 8
D2d, [4,2+], (2*2)
Order 8
D3h, [3,2], (*223)
Order 12

basketball seam paths

baseball seam paths

Beach ball
(ignoring colors)

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


D5h, [5], (*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

See also


External links

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