- Tetrahedral symmetry
A regular

tetrahedron has 12 rotational (or orientation-preserving) symmetries, and a total of 24 symmetries including transformations that combine a reflection and a rotation.The group of symmetries that includes reflections is isomorphic to "S"

_{4}, or the group of permutations of four objects, since there is exactly one such symmetry for each permutation of the vertices of the tetrahedron. The set of orientation-preserving symmetries forms a group referred to as "A"_{4}.**Details****Chiral**and**full**(or**achiral) tetrahedral symmetry**and**pyritohedral symmetry**are discrete point symmetries (or equivalently, symmetries on the sphere). They are among the crystallographic point groups of the cubic crystal system.**Chiral tetrahedral symmetry****"T**" or**332**or**23**, of order 12 -**chiral**or**rotational tetrahedral symmetry**. There are three orthogonal 2-fold rotation axes, like chiral dihedral symmetry "D"_{2}or 222, with in addition four 3-fold axes, centered "between" the three orthogonal directions. This group isisomorphic to "A"_{4}, thealternating group on 4 elements; in fact it is the group ofeven permutation s of the four 3-fold axes: e, (123), (132), (124), (142), (134), (143), (234), (243), (12)(34), (13)(24), (14)(23).The

conjugacy class es of "T" are:

*identity

*4 × rotation by 120° clockwise (seen from a vertex): (234), (143), (412), (321)

*4 × rotation by 120° anti-clockwise (ditto)

*3 × rotation by 180°The rotations by 180°, together with the identity, form a

normal subgroup of type Dih_{2}, withquotient group of type Z_{3}. The three elements of the latter are the identity, "clockwise rotation", and "anti-clockwise rotation", corresponding to permutations of the three orthogonal 2-fold axes, preserving orientation."A"

_{4}is the smallest group demonstrating that the converse of Lagrange's theorem is not true in general: given a finite group "G" and a divisor "d" of |"G"|, there does not necessarily exist a subgroup of "G" with order "d": the group "G" = "A"_{4 has no subgroup of order 6. Although it is a property for the abstract group in general, it is clear from the isometry group of chiral tetrahedral symmetry: because of the chirality the subgroup would have to be C6 or D3, but neither applies.}**Achiral tetrahedral symmetry****"T**" or_{d}***332**or $ar\{4\}3m$, of order 24 -**achiral**or**full tetrahedral symmetry**. This group has the same rotation axes as "T", but with six mirror planes, each through two 3-fold axes. The 2-fold axes are now "S"_{4}($ar\{4\}$) axes. "T_{d}" and "O" are isomorphic as abstract groups: they both correspond to "S"_{4}, thesymmetric group on 4 objects. "T_{d}" is the union of "T" and the set obtained by combining each element of "O" "T" with inversion. See also the isometries of the regular tetrahedron.The conjugacy classes of "T

_{d}" are:

*identity

*8 × rotation by 120°

*3 × rotation by 180°

*6 × reflection in a plane through two rotation axes

*6 × rotoreflection by 90°**Pyritohedral symmetry****"T"**or_{h}**3*2**or $2/m\; ar\{3\}$, of order 24 -**pyritohedral symmetry**. This group has the same rotation axes as "T", with mirror planes through two of the orthogonal directions. The 3-fold axes are now S_{6}($ar\{3\}$) axes, and there is inversion symmetry. "T"_{h}is isomorphic to "T" × "Z"_{2}: every element of "T"_{h}is either an element of "T", or one combined with inversion. Apart from these two normal subgroups, there is also a normal subgroup "D_{2h}" (that of acuboid ), of type Dih_{2}× "Z_{2}" = "Z_{2}" × "Z_{2}" × "Z_{2}" . It is the direct product of the normal subgroup of "T" (see above) with "C_{i}". Thequotient group is the same as above: of type Z_{3}. The three elements of the latter are the identity, "clockwise rotation", and "anti-clockwise rotation", corresponding to permutations of the three orthogonal 2-fold axes, preserving orientation.It is the symmetry of a cube with on each face a line segment dividing the face into two equal rectangles, such that the line segments of adjacent faces do not meet at the edge. The symmetries correspond to the even permutations of the body diagonals and the same combined with inversion. It is also the symmetry of a

pyritohedron , which is similar to the cube described, with each rectangle replaced by a pentagon with one symmetry axis and 4 equal sides and 1 different side (the one corresponding to the line segment dividing the cube's face); i.e., the cube's faces bulge out at the dividing line and become narrower there. It is a subgroup of the fullicosahedral symmetry group (as isometry group, not just as abstract group), with 4 of the 10 3-fold axes.The conjugacy classes of "T

_{h}" include those of "T", with the two classes of 4 combined, and each with inversion:

*identity

*8 × rotation by 120°

*3 × rotation by 180°

*inversion

*8 × rotoreflection by 60°

*3 × reflection in a plane**Solids with chiral tetrahedral symmetry**The Icosahedron colored as a

**snub tetrahedron**has chiral symmetry.**olids with full tetrahedral symmetry**Platonic solid :Archimedean solid :(semi-regular: vertex-uniform)

Catalan solid :(semi-regular dual: face-uniform)

Name picture Dual Archimedean solid Faces Edges Vertices Face polygon triakis tetrahedron ) truncated tetrahedron 12 18 8 isosceles triangle **ee also***

octahedral symmetry

*icosahedral symmetry

*binary tetrahedral group

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