5-cell

When a net of five tetrahedra is folded up in 4-dimensional space such that each tetrahedron is face bonded to the other four, the resulting 5-cell has a total of 5 vertices, 10 edges and 10 faces. Four edges meet at each vertex, and three tetrahedral cells meet at each edge.

The 5-cell is self-dual (as are all simplexes), and its vertex figure is the tetrahedron.[lower-alpha 5] Its maximal intersection with 3-dimensional space is the triangular prism. Its dihedral angle is cos⁻¹(1/4), or approximately 75.52°.

The convex hull of two 5-cells in dual configuration is the disphenoidal 30-cell, dual of the bitruncated 5-cell.

This configuration matrix represents the 5-cell. The rows and columns correspond to vertices, edges, faces, and cells. The diagonal numbers say how many of each element occur in the whole 5-cell. The nondiagonal numbers say how many of the column's element occur in or at the row's element. This self-dual polytope's matrix is identical to its 180 degree rotation.[7] The k-faces can be read as rows left of the diagonal, while the k-figures are read as rows after the diagonal.[8]

Grünbaum's rotationally symmetrical 5-set Venn diagram, 1975

Element	k-face	fk	f0	f1	f2	f3	k-figs
	( )	f0	5	4	6	4	{3,3}
	{ }	f1	2	10	3	3	{3}
	{3}	f2	3	3	10	2	{ }
	{3,3}	f3	4	6	4	5	( )

All these elements of the 5-cell are enumerated in Branko Grünbaum's Venn diagram of 5 points, which is literally an illustration of the regular 5-cell in projection to the plane.

The simplest set of Cartesian coordinates is: (2,0,0,0), (0,2,0,0), (0,0,2,0), (0,0,0,2), (φ,φ,φ,φ), with edge length 2√2, where φ is the golden ratio.[9] While these coordinates are not origin-centered, subtracting ${\displaystyle (1,1,1,1)/(2-{\tfrac {1}{\varphi }})}$ from each translates the 4-polytope's circumcenter to the origin with radius ${\displaystyle 2(\varphi -1/(2-{\tfrac {1}{\varphi }}))={\sqrt {\tfrac {16}{5}}}\approx 1.7888}$, with the following coordinates:

${\displaystyle \left({\tfrac {2}{\varphi }}-3,1,1,1\right)/({\tfrac {1}{\varphi }}-2)}$

${\displaystyle \left(1,{\tfrac {2}{\varphi }}-3,1,1\right)/({\tfrac {1}{\varphi }}-2)}$

${\displaystyle \left(1,1,{\tfrac {2}{\varphi }}-3,1\right)/({\tfrac {1}{\varphi }}-2)}$

${\displaystyle \left(1,1,1,{\tfrac {2}{\varphi }}-3\right)/({\tfrac {1}{\varphi }}-2)}$

${\displaystyle \left({\tfrac {2}{\varphi }},{\tfrac {2}{\varphi }},{\tfrac {2}{\varphi }},{\tfrac {2}{\varphi }}\right)/({\tfrac {1}{\varphi }}-2)}$

The following set of origin-centered coordinates with the same radius and edge length as above can be seen as a hyperpyramid with a regular tetrahedral base in 3-space:

${\displaystyle \left(1,1,1,{\frac {-1}{\sqrt {5}}}\right)}$

${\displaystyle \left(1,-1,-1,{\frac {-1}{\sqrt {5}}}\right)}$

${\displaystyle \left(-1,1,-1,{\frac {-1}{\sqrt {5}}}\right)}$

${\displaystyle \left(-1,-1,1,{\frac {-1}{\sqrt {5}}}\right)}$

${\displaystyle \left(0,0,0,{\frac {4}{\sqrt {5}}}\right)}$

Scaling these or the previous set of coordinates by ${\displaystyle {\tfrac {\sqrt {5}}{4}}}$ give unit-radius origin-centered regular 5-cells with edge lengths ${\displaystyle {\sqrt {\tfrac {5}{2}}}}$. The hyperpyramid has coordinates:

${\displaystyle \left({\sqrt {5}},{\sqrt {5}},{\sqrt {5}},-1\right)/4}$

${\displaystyle \left({\sqrt {5}},-{\sqrt {5}},-{\sqrt {5}},-1\right)/4}$

${\displaystyle \left(-{\sqrt {5}},{\sqrt {5}},-{\sqrt {5}},-1\right)/4}$

${\displaystyle \left(-{\sqrt {5}},-{\sqrt {5}},{\sqrt {5}},-1\right)/4}$

${\displaystyle \left(0,0,0,1\right)}$

Coordinates for the vertices of another origin-centered regular 5-cell with edge length 2 and radius ${\displaystyle {\sqrt {\tfrac {8}{5}}}\approx 1.265}$ are:

${\displaystyle \left({\frac {1}{\sqrt {10}}},\ {\frac {1}{\sqrt {6}}},\ {\frac {1}{\sqrt {3}}},\ \pm 1\right)}$

${\displaystyle \left({\frac {1}{\sqrt {10}}},\ {\frac {1}{\sqrt {6}}},\ {\frac {-2}{\sqrt {3}}},\ 0\right)}$

${\displaystyle \left({\frac {1}{\sqrt {10}}},\ -{\sqrt {\frac {3}{2}}},\ 0,\ 0\right)}$

${\displaystyle \left(-2{\sqrt {\frac {2}{5}}},\ 0,\ 0,\ 0\right)}$

Scaling these by ${\displaystyle {\sqrt {\tfrac {5}{8}}}}$ to unit-radius and edge length ${\displaystyle {\sqrt {\tfrac {5}{2}}}}$ gives:

${\displaystyle \left({\sqrt {3}},{\sqrt {5}},{\sqrt {10}},\pm {\sqrt {30}}\right)/(4{\sqrt {3}})}$

${\displaystyle \left({\sqrt {3}},{\sqrt {5}},-{\sqrt {40}},0\right)/(4{\sqrt {3}})}$

${\displaystyle \left({\sqrt {3}},-{\sqrt {45}},0,0\right)/(4{\sqrt {3}})}$

${\displaystyle \left(-1,0,0,0\right)}$

The vertices of a 4-simplex (with edge √2 and radius 1) can be more simply constructed on a hyperplane in 5-space, as (distinct) permutations of (0,0,0,0,1) or (0,1,1,1,1); in these positions it is a facet of, respectively, the 5-orthoplex or the rectified penteract.

The 5-cell has only digon central planes through vertices, no two of which are orthogonal. It has 10 digon central planes, where each vertex pair is an edge, not an axis, of the 5-cell.[lower-alpha 4] The characteristic isoclinic rotation of the 5-cell has, as pairs of invariant planes, those 10 digon planes and their completely orthogonal central planes, which are 0-gon planes which intersect no 5-cell vertices.

There are exactly three different ways to make a circuit of the 5-cell through all 5 vertices along 5 edges,[lower-alpha 5] so there are three discrete Hopf fibrations of the great digons of the 5-cell. Each of the three fibrations corresponds to a distinct left-right pair of isoclinic rotations which each rotate all 5 vertices in a circuit of period 5. The 5-cell has only three distinct period 5 isoclines (those circles through all 5 vertices), each of which acts as the single isocline of a right rotation and the single isocline of a left rotation in two different fibrations, and as the Petrie polygon of the 5-cell in the third fibration.[lower-alpha 6]

A 5-cell can be constructed as a Boerdijk–Coxeter helix of five chained tetrahedra, folded into a 4-dimensional ring.[10] The 10 triangle faces can be seen in a 2D net within a triangular tiling, with 6 triangles around every vertex, although folding into 4-dimensions causes edges to coincide. The purple edges form a regular pentagon which is the Petrie polygon of the 5-cell. The blue edges connect every second vertex, forming a pentagram which is the Clifford polygon of the 5-cell. The pentagram's blue edges are the chords of the 5-cell's isocline, the circular rotational path its vertices take during an isoclinic rotation, also known as a Clifford displacement.[lower-alpha 8]

The A₄ Coxeter plane projects the 5-cell into a regular pentagon and pentagram. The A₃ Coxeter plane projection of the 5-cell is that of a square pyramid. The A₂ Coxeter plane projection of the regular 5-cell is that of a triangular bipyramid (two tetrahedra joined face-to-face) with the two opposite vertices centered.

orthographic projections

Ak Coxeter plane	A4	A3	A2
Graph
Dihedral symmetry	[5]	[4]	[3]

Projections to 3 dimensions
The vertex-first projection of the 5-cell into 3 dimensions has a tetrahedral projection envelope. The closest vertex of the 5-cell projects to the center of the tetrahedron, as shown here in red. The farthest cell projects onto the tetrahedral envelope itself, while the other 4 cells project onto the 4 flattened tetrahedral regions surrounding the central vertex.	The edge-first projection of the 5-cell into 3 dimensions has a triangular dipyramidal envelope. The closest edge (shown here in red) projects to the axis of the dipyramid, with the three cells surrounding it projecting to 3 tetrahedral volumes arranged around this axis at 120 degrees to each other. The remaining 2 cells project to the two halves of the dipyramid and are on the far side of the pentatope.
The face-first projection of the 5-cell into 3 dimensions also has a triangular dipyramidal envelope. The nearest face is shown here in red. The two cells that meet at this face project to the two halves of the dipyramid. The remaining three cells are on the far side of the pentatope from the 4D viewpoint, and are culled from the image for clarity. They are arranged around the central axis of the dipyramid, just as in the edge-first projection.	The cell-first projection of the 5-cell into 3 dimensions has a tetrahedral envelope. The nearest cell projects onto the entire envelope, and, from the 4D viewpoint, obscures the other 4 cells; hence, they are not rendered here.

Stereographic projection wireframe (edge projected onto a 3-sphere)

A 4-orthoscheme is a 5-cell where all 10 faces are right triangles.[lower-alpha 1] An orthoscheme is an irregular simplex that is the convex hull of a tree in which all edges are mutually perpendicular. In a 4-dimensional orthoscheme, the tree consists of four perpendicular edges connecting all five vertices in a linear path that makes three right-angled turns. The elements of an orthoscheme are also orthoschemes (just as the elements of a regular simplex are also regular simplexes). Each tetrahedral cell of a 4-orthoscheme is a 3-orthoscheme, and each triangular face is a 2-orthoscheme (a right triangle).

Orthoschemes are the characteristic simplexes of the regular polytopes, because each regular polytope is generated by reflections in the bounding facets of its particular characteristic orthoscheme.[11] For example, the special case of the 4-orthoscheme with equal-length perpendicular edges is the characteristic orthoscheme of the 4-cube (also called the tesseract or 8-cell), the 4-dimensional analogue of the 3-dimensional cube. If the three perpendicular edges of the 4-orthoscheme are of unit length, then all its edges are of length √1, √2, √3, or √4, precisely the chord lengths of the unit 4-cube (the lengths of the 4-cube's edges and its various diagonals). Therefore this 4-orthoscheme fits within the 4-cube, and the 4-cube (like every regular convex polytope) can be dissected into instances of its characteristic orthoscheme.

A 3-cube dissected into six 3-orthoschemes. Three are left-handed and three are right handed. A left and a right meet at each square face.

A 3-orthoscheme is easily illustrated, but a 4-orthoscheme is more difficult to visualize. A 4-orthoscheme is a tetrahedral pyramid with a 3-orthoscheme as its base. It has four more edges than the 3-orthoscheme, joining the four vertices of the base to its apex (the fifth vertex of the 5-cell). Pick out any one of the 3-orthoschemes of the six shown in the 3-cube illustration. Notice that it touches four of the cube's eight vertices, and those four vertices are linked by a 3-edge path that makes two right-angled turns. Imagine that this 3-orthoscheme is the base of a 4-orthoscheme, so that from each of those four vertices, an unseen 4-orthoscheme edge connects to a fifth apex vertex (which is outside the 3-cube and does not appear in the illustration at all). Although the four additional edges all reach the same apex vertex, they will all be of different lengths. The first of them, at one end of the 3-edge orthogonal path, extends that path with a fourth orthogonal √1 edge by making a third 90 degree turn and reaching perpendicularly into the fourth dimension to the apex. The second of the four additional edges is a √2 diagonal of a cube face (not of the illustrated 3-cube, but of another of the tesseract's eight 3-cubes).[lower-alpha 9] The third additional edge is a √3 diagonal of a 3-cube (again, not the original illustrated 3-cube). The fourth additional edge (at the other end of the orthogonal path) is a long diameter of the tesseract itself, of length √4. It reaches through the exact center of the tesseract to the antipodal vertex (a vertex of the opposing 3-cube), which is the apex. Thus the characteristic 5-cell of the 4-cube has four √1 edges, three √2 edges, two √3 edges, and one √4 edge.

The 4-cube can be dissected into 24 such 4-orthoschemes eight different ways, with six 4-orthoschemes surrounding each of four orthogonal √4 tesseract long diameters. The 4-cube can also be dissected into 384 smaller instances of this same characteristic 4-orthoscheme, just one way, by all of its symmetry hyperplanes at once, which divide it into 384 4-orthoschemes that all meet at the center of the 4-cube.[lower-alpha 10]

More generally, any regular polytope can be dissected into g instances of its characteristic orthoscheme that all meet at the regular polytope's center.[12] The number g is the order of the polytope, the number of reflected instances of its characteristic orthoscheme that comprise the polytope when a single mirror-surfaced orthoscheme instance is reflected in its own facets.[lower-alpha 11] More generally still, characteristic simplexes are able to fill uniform polytopes because they possess all the requisite elements of the polytope. They also possess all the requisite angles between elements (from 90 degrees on down). The characteristic simplexes are the genetic codes of polytopes: like a Swiss Army knife, they contain one of everything needed to construct the polytope by replication.

Every regular polytope, including the regular 5-cell, has its characteristic orthoscheme.[lower-alpha 12] There is a 4-orthoscheme which is the characteristic 5-cell of the regular 5-cell. It is a tetrahedral pyramid based on the characteristic tetrahedron of the regular tetrahedron. The regular 5-cell can be dissected into 120 instances of this characteristic 4-orthoscheme just one way, by all of its symmetry hyperplanes at once, which divide it into 120 4-orthoschemes that all meet at the center of the regular 5-cell.[lower-alpha 13]

Characteristics of the regular 5-cell[16]
	edge[17]	arc		dihedral[18]
𝒍	${\displaystyle {\sqrt {\tfrac {5}{2}}}\approx 1.581}$	104°30′40″	${\displaystyle \pi -2{\text{𝜂}}}$	75°29′20″	${\displaystyle \pi -2{\text{𝟁}}}$
𝟀	${\displaystyle {\sqrt {\tfrac {1}{10}}}\approx 0.316}$	75°29′20″	${\displaystyle 2{\text{𝜂}}}$	60°	${\displaystyle {\tfrac {\pi }{3}}}$
𝝓	${\displaystyle {\sqrt {\tfrac {1}{30}}}\approx 0.183}$	52°15′20″	${\displaystyle {\tfrac {\pi }{2}}-{\text{𝜂}}}$	60°	${\displaystyle {\tfrac {\pi }{3}}}$
𝟁	${\displaystyle {\sqrt {\tfrac {2}{15}}}\approx 0.103}$	52°15′20″	${\displaystyle {\tfrac {\pi }{2}}-{\text{𝜂}}}$	60°	${\displaystyle {\tfrac {\pi }{3}}}$
${\displaystyle _{0}R^{3}/l}$	${\displaystyle {\sqrt {\tfrac {3}{20}}}\approx 0.387}$	75°29′20″	${\displaystyle 2{\text{𝜂}}}$	90°	${\displaystyle {\tfrac {\pi }{2}}}$
${\displaystyle _{1}R^{3}/l}$	${\displaystyle {\sqrt {\tfrac {1}{20}}}\approx 0.224}$	52°15′20″	${\displaystyle {\tfrac {\pi }{2}}-{\text{𝜂}}}$	90°	${\displaystyle {\tfrac {\pi }{2}}}$
${\displaystyle _{2}R^{3}/l}$	${\displaystyle {\sqrt {\tfrac {1}{60}}}\approx 0.129}$	52°15′20″	${\displaystyle {\tfrac {\pi }{2}}-{\text{𝜂}}}$	90°	${\displaystyle {\tfrac {\pi }{2}}}$
${\displaystyle _{0}R^{4}/l}$	${\displaystyle {\sqrt {\tfrac {4}{25}}}=0.4}$
${\displaystyle _{1}R^{4}/l}$	${\displaystyle {\sqrt {\tfrac {3}{50}}}\approx 0.245}$
${\displaystyle _{2}R^{4}/l}$	${\displaystyle {\sqrt {\tfrac {2}{43}}}\approx 0.216}$
${\displaystyle _{3}R^{4}/l}$	${\displaystyle {\sqrt {\tfrac {1}{100}}}=0.1}$
${\displaystyle {\text{𝜼}}}$		37°44′40″	${\displaystyle {\tfrac {{\text{arc sec }}4}{2}}}$

The characteristic 5-cell (4-orthoscheme) of the regular 5-cell has four more edges than its base characteristic tetrahedron (3-orthoscheme), which join the four vertices of the base to its apex (the fifth vertex of the 4-orthoscheme, at the center of the regular 5-cell).[lower-alpha 14] If the regular 5-cell has unit radius and edge length 𝒍 = ${\displaystyle {\sqrt {\tfrac {5}{2}}}}$, its characteristic 5-cell's ten edges have lengths ${\displaystyle {\sqrt {\tfrac {1}{10}}}}$, ${\displaystyle {\sqrt {\tfrac {1}{30}}}}$, ${\displaystyle {\sqrt {\tfrac {2}{15}}}}$ (the exterior right triangle face, the characteristic triangle 𝟀, 𝝓, 𝟁), plus ${\displaystyle {\sqrt {\tfrac {3}{20}}}}$, ${\displaystyle {\sqrt {\tfrac {1}{20}}}}$, ${\displaystyle {\sqrt {\tfrac {1}{60}}}}$ (the other three edges of the exterior 3-orthoscheme facet the characteristic tetrahedron, which are the characteristic radii of the regular tetrahedron), plus ${\displaystyle {\sqrt {\tfrac {4}{25}}}}$, ${\displaystyle {\sqrt {\tfrac {3}{50}}}}$, ${\displaystyle {\sqrt {\tfrac {2}{43}}}}$, ${\displaystyle {\sqrt {\tfrac {1}{100}}}}$ (edges which are the characteristic radii of the regular 5-cell). The 4-edge path along orthogonal edges of the orthoscheme is ${\displaystyle {\sqrt {\tfrac {1}{30}}}}$, ${\displaystyle {\sqrt {\tfrac {2}{15}}}}$, ${\displaystyle {\sqrt {\tfrac {1}{60}}}}$, ${\displaystyle {\sqrt {\tfrac {1}{100}}}}$, first from a regular 5-cell vertex to a regular 5-cell edge center, then turning 90° to a regular 5-cell face center, then turning 90° to a regular 5-cell tetrahedral cell center, then turning 90° to the regular 5-cell center.[lower-alpha 15]

There are many lower symmetry forms of the 5-cell, including these found as uniform polytope vertex figures:

Symmetry	[3,3,3] Order 120	[3,3,1] Order 24	[3,2,1] Order 12	[3,1,1] Order 6	~[5,2]+ Order 10
Name	Regular 5-cell	Tetrahedral pyramid		Triangular pyramidal pyramid
Schläfli	{3,3,3}	{3,3}∨( )	{3}∨{ }	{3}∨( )∨( )
Example Vertex figure	5-simplex	Truncated 5-simplex	Bitruncated 5-simplex	Cantitruncated 5-simplex	Omnitruncated 4-simplex honeycomb

The tetrahedral pyramid is a special case of a 5-cell, a polyhedral pyramid, constructed as a regular tetrahedron base in a 3-space hyperplane, and an apex point above the hyperplane. The four sides of the pyramid are made of triangular pyramid cells.

Many uniform 5-polytopes have tetrahedral pyramid vertex figures with Schläfli symbols ( )∨{3,3}.

Symmetry [3,3,1], order 24

Schlegel diagram
Name Coxeter	{ }×{3,3,3}	{ }×{4,3,3}	{ }×{5,3,3}	t{3,3,3,3}	t{4,3,3,3}	t{3,4,3,3}

Other uniform 5-polytopes have irregular 5-cell vertex figures. The symmetry of a vertex figure of a uniform polytope is represented by removing the ringed nodes of the Coxeter diagram.

Symmetry	[3,2,1], order 12		[3,1,1], order 6		[2+,4,1], order 8	[2,1,1], order 4
Schläfli	{3}∨{  }		{3}∨( )∨( )		{ }∨{ }∨( )
Schlegel diagram
Name Coxeter	t12α5	t12γ5	t012α5	t012γ5	t123α5	t123γ5

Symmetry	[2,1,1], order 2			[2+,1,1], order 2	[ ]+, order 1
Schläfli	{ }∨( )∨( )∨( )			( )∨( )∨( )∨( )∨( )
Schlegel diagram
Name Coxeter	t0123α5	t0123γ5	t0123β5	t01234α5	t01234γ5