Editing 16-cell (section)

=== Constructions ===

==== Octahedral dipyramid ====
{|class="wikitable floatright"
!Octahedron <math>\beta_3</math>
!16-cell <math>\beta_4</math>
|-
|[[File:3-cube t2.svg|160px]]
|[[File:4-demicube t0 D4.svg|160px]]
|-
|colspan=2|Orthogonal projections to skew hexagon hyperplane
|}
The simplest construction of the 16-cell is on the 3-dimensional cross polytope, the [[octahedron]]. The octahedron has 3 perpendicular axes and 6 vertices in 3 opposite pairs (its [[Petrie polygon]] is the [[hexagon]]). Add another pair of vertices, on a fourth axis perpendicular to all 3 of the other axes. Connect each new vertex to all 6 of the original vertices, adding 12 new edges. This raises two [[octahedral pyramid]]s on a shared octahedron base that lies in the 16-cell's central hyperplane.{{Sfn|Coxeter|1973|p=121|loc=§&nbsp;7.21. See illustration Fig 7.2<small>B</small>|ps=: "<math>\beta_4</math> is a four-dimensional dipyramid based on <math>\beta_3</math> (with its two apices in opposite directions along the fourth dimension)."}}

[[File:stereographic_polytope_16cell_colour.png|thumb|[[Stereographic projection]] of the 16-cell's 6 orthogonal central squares onto their great circles. Each circle is divided into 4 arc-edges at the intersections where 3 circles cross perpendicularly. Notice that each circle has one Clifford parallel circle that it does ''not'' intersect. Those two circles pass through each other like adjacent links in a chain.]]The octahedron that the construction starts with has three perpendicular intersecting squares (which appear as rectangles in the hexagonal projections). Each square intersects with each of the other squares at two opposite vertices, with ''two'' of the squares crossing at each vertex. Then two more points are added in the fourth dimension (above and below the 3-dimensional hyperplane). These new vertices are connected to all the octahedron's vertices, creating 12 new edges and ''three more squares'' (which appear edge-on as the 3 ''diameters'' of the hexagon in the projection), and three more octahedra.{{Efn|name=octahedral hyperplanes}}

Something unprecedented has also been created. Notice that each square no longer intersects with ''all'' of the other squares: it does intersect with four of them (with ''three'' of the squares crossing at each vertex now), but each square has ''one'' other square with which it shares ''no'' vertices: it is not directly connected to that square at all. These two ''separate'' perpendicular squares (there are three pairs of them) are like the opposite edges of a [[tetrahedron]]: perpendicular, but non-intersecting. They lie opposite each other (parallel in some sense), and they don't touch, but they also pass through each other like two perpendicular links in a chain (but unlike links in a chain they have a common center). They are an example of '''''Clifford parallel planes''''', and the 16-cell is the simplest regular polytope in which they occur. [[William Kingdon Clifford|Clifford]] parallelism{{Efn|name=Clifford parallels}} of objects of more than one dimension (more than just curved ''lines'') emerges here and occurs in all the subsequent 4-dimensional regular polytopes, where it can be seen as the defining relationship ''among'' disjoint concentric regular 4-polytopes and their corresponding parts. It can occur between congruent (similar) polytopes of 2 or more dimensions.{{Sfn|Tyrrell|Semple|1971}} For example, as noted [[#Geometry|above]] all the subsequent convex regular 4-polytopes are compounds of multiple 16-cells; those 16-cells are [[24-cell#Clifford parallel polytopes|Clifford parallel polytopes]].

==== Tetrahedral constructions ====
{| class="wikitable" width=480
|- align=center valign=top
|[[File:16-cell net.png|180px|]]
|[[File:16-cell nets.png|180px]]
|}

The 16-cell has two [[Wythoff construction]]s from regular tetrahedra, a regular form and alternated form, shown here as [[Net (polyhedron)|nets]], the second represented by tetrahedral cells of two alternating colors. The alternated form is a [[#Symmetry constructions|lower symmetry construction]] of the 16-cell called the [[demitesseract]].

Wythoff's construction replicates the 16-cell's [[5-cell#Orthoschemes|characteristic 5-cell]] in a [[kaleidoscope]] of mirrors. Every regular 4-polytope has its characteristic 4-orthoscheme, an [[5-cell#Irregular 5-cells|irregular 5-cell]].{{Efn|An [[orthoscheme]] is a [[chiral]] irregular [[simplex]] with [[right triangle]] faces that is characteristic of some polytope if it will exactly fill that polytope with the reflections of itself in its own [[facet (geometry)|facets]] (its ''mirror walls''). Every regular polytope can be dissected radially into instances of its [[Orthoscheme#Characteristic simplex of the general regular polytope|characteristic orthoscheme]] surrounding its center. The characteristic orthoscheme has the shape described by the same [[Coxeter-Dynkin diagram]] as the regular polytope without the ''generating point'' ring.|name=characteristic orthoscheme}} There are three regular 4-polytopes with tetrahedral cells: the [[5-cell]], the 16-cell, and the [[600-cell]]. Although all are bounded by ''regular'' tetrahedron cells, their characteristic 5-cells (4-orthoschemes) are different [[5-cell#Isometries|tetrahedral pyramids]], all based on the same characteristic ''irregular'' tetrahedron. They share the same [[Tetrahedron#Orthoschemes|characteristic tetrahedron]] (3-orthoscheme) and characteristic [[right triangle]] (2-orthoscheme) because they have the same kind of cell.{{Efn|A regular polytope of dimension ''k'' has a characteristic ''k''-orthoscheme, and also a characteristic (''k''-1)-orthoscheme. A regular 4-polytope has a characteristic 5-cell (4-orthoscheme) into which it is subdivided by its (3-dimensional) hyperplanes of symmetry, and also a characteristic tetrahedron (3-orthoscheme) into which its surface is subdivided by its cells' (2-dimensional) planes of symmetry. After subdividing its (3-dimensional) surface into characteristic tetrahedra surrounding each cell center, its (4-dimensional) interior can be subdivided into characteristic 5-cells by adding radii joining the vertices of the surface characteristic tetrahedra to the 4-polytope's center.{{Sfn|Coxeter|1973|p=130|loc=§&nbsp;7.6|ps=; "simplicial subdivision".}} The interior tetrahedra and triangles thus formed will also be orthoschemes.}}

{| class="wikitable floatright"
!colspan=6|Characteristics of the 16-cell{{Sfn|Coxeter|1973|pp=292-293|loc=Table I(ii); "16-cell, 𝛽<sub>4</sub>"}}
|-
!align=right|
!align=center|edge{{Sfn|Coxeter|1973|p=139|loc=§&nbsp;7.9 The characteristic simplex}}
!colspan=2 align=center|arc
!colspan=2 align=center|dihedral{{Sfn|Coxeter|1973|p=290|loc=Table I(ii); "dihedral angles"}}
|-
!align=right|𝒍
|align=center|<small><math>\sqrt{2} \approx 1.414</math></small>
|align=center|<small>90°</small>
|align=center|<small><math>\tfrac{\pi}{2}</math></small>
|align=center|<small>120°</small>
|align=center|<small><math>\tfrac{2\pi}{3}</math></small>
|-
|
|
|
|
|
|-
!align=right|𝟀
|align=center|<small><math>\sqrt{\tfrac{2}{3}} \approx 0.816</math></small>
|align=center|<small>60″</small>
|align=center|<small><math>\tfrac{\pi}{3}</math></small>
|align=center|<small>60°</small>
|align=center|<small><math>\tfrac{\pi}{3}</math></small>
|-
!align=right|𝝉{{Efn|{{Harv|Coxeter|1973}} uses the greek letter 𝝓 (phi) to represent one of the three ''characteristic angles'' 𝟀, 𝝓, 𝟁 of a regular polytope. Because 𝝓 is commonly used to represent the [[golden ratio]] constant ≈ 1.618, for which Coxeter uses 𝝉 (tau), we reverse Coxeter's conventions, and use 𝝉 to represent the characteristic angle.|name=reversed greek symbols}}
|align=center|<small><math>\sqrt{\tfrac{1}{2}} \approx 0.707</math></small>
|align=center|<small>45″</small>
|align=center|<small><math>\tfrac{\pi}{4}</math></small>
|align=center|<small>45°</small>
|align=center|<small><math>\tfrac{\pi}{4}</math></small>
|-
!align=right|𝟁
|align=center|<small><math>\sqrt{\tfrac{1}{6}} \approx 0.408</math></small>
|align=center|<small>30″</small>
|align=center|<small><math>\tfrac{\pi}{6}</math></small>
|align=center|<small>60°</small>
|align=center|<small><math>\tfrac{\pi}{3}</math></small>
|-
|
|
|
|
|
|-
!align=right|<small><math>_0R^3/l</math></small>
|align=center|<small><math>\sqrt{\tfrac{3}{4}} \approx 0.866</math></small>
|align=center|<small>60°</small>
|align=center|<small><math>\tfrac{\pi}{3}</math></small>
|align=center|<small>90°</small>
|align=center|<small><math>\tfrac{\pi}{2}</math></small>
|-
!align=right|<small><math>_1R^3/l</math></small>
|align=center|<small><math>\sqrt{\tfrac{1}{4}} = 0.5</math></small>
|align=center|<small>45°</small>
|align=center|<small><math>\tfrac{\pi}{4}</math></small>
|align=center|<small>90°</small>
|align=center|<small><math>\tfrac{\pi}{2}</math></small>
|-
!align=right|<small><math>_2R^3/l</math></small>
|align=center|<small><math>\sqrt{\tfrac{1}{12}} \approx 0.289</math></small>
|align=center|<small>30°</small>
|align=center|<small><math>\tfrac{\pi}{6}</math></small>
|align=center|<small>90°</small>
|align=center|<small><math>\tfrac{\pi}{2}</math></small>
|-
|
|
|
|
|
|-
!align=right|<small><math>_0R^4/l</math></small>
|align=center|<small><math>1</math></small>
|align=center|
|align=center|
|align=center|
|align=center|
|-
!align=right|<small><math>_1R^4/l</math></small>
|align=center|<small><math>\sqrt{\tfrac{1}{2}} \approx 0.707</math></small>
|align=center|
|align=center|
|align=center|
|align=center|
|-
!align=right|<small><math>_2R^4/l</math></small>
|align=center|<small><math>\sqrt{\tfrac{1}{3}} \approx 0.577</math></small>
|align=center|
|align=center|
|align=center|
|align=center|
|-
!align=right|<small><math>_3R^4/l</math></small>
|align=center|<small><math>\sqrt{\tfrac{1}{4}} = 0.5</math></small>
|align=center|
|align=center|
|align=center|
|align=center|
|}
The '''characteristic 5-cell of the regular 16-cell''' is represented by the [[Coxeter-Dynkin diagram]] {{CDD|node|3|node|3|node|4|node}}, which can be read as a list of the dihedral angles between its mirror facets. It is an irregular [[Pyramid (mathematics)#Polyhedral pyramid|tetrahedral pyramid]] based on the [[Tetrahedron#Orthoschemes|characteristic tetrahedron of the regular tetrahedron]]. The regular 16-cell is subdivided by its symmetry hyperplanes into 384 instances of its characteristic 5-cell that all meet at its center.

The characteristic 5-cell (4-orthoscheme) has four more edges than its base characteristic tetrahedron (3-orthoscheme), joining the four vertices of the base to its apex (the fifth vertex of the 4-orthoscheme, at the center of the regular 16-cell).{{Efn|The four edges of each 4-orthoscheme which meet at the center of a regular 4-polytope are of unequal length, because they are the four characteristic radii of the regular 4-polytope: a vertex radius, an edge center radius, a face center radius, and a cell center radius. The five vertices of the 4-orthoscheme always include one regular 4-polytope vertex, one regular 4-polytope edge center, one regular 4-polytope face center, one regular 4-polytope cell center, and the regular 4-polytope center. Those five vertices (in that order) comprise a path along four mutually perpendicular edges (that makes three right angle turns), the characteristic feature of a 4-orthoscheme. The 4-orthoscheme has five dissimilar 3-orthoscheme facets.|name=characteristic radii}} If the regular 16-cell has unit radius edge and edge length 𝒍 = <small><math>\sqrt{2}</math></small>, its characteristic 5-cell's ten edges have lengths <small><math>\sqrt{\tfrac{2}{3}}</math></small>, <small><math>\sqrt{\tfrac{1}{2}}</math></small>, <small><math>\sqrt{\tfrac{1}{6}}</math></small> around its exterior right-triangle face (the edges opposite the ''characteristic angles'' 𝟀, 𝝉, 𝟁),{{Efn|name=reversed greek symbols}} plus <small><math>\sqrt{\tfrac{3}{4}}</math></small>, <small><math>\sqrt{\tfrac{1}{4}}</math></small>, <small><math>\sqrt{\tfrac{1}{12}}</math></small> (the other three edges of the exterior 3-orthoscheme facet the characteristic tetrahedron, which are the ''characteristic radii'' of the regular tetrahedron), plus <small><math>1</math></small>, <small><math>\sqrt{\tfrac{1}{2}}</math></small>, <small><math>\sqrt{\tfrac{1}{3}}</math></small>, <small><math>\sqrt{\tfrac{1}{4}}</math></small> (edges which are the characteristic radii of the regular 16-cell). The 4-edge path along orthogonal edges of the orthoscheme is <small><math>\sqrt{\tfrac{1}{2}}</math></small>, <small><math>\sqrt{\tfrac{1}{6}}</math></small>, <small><math>\sqrt{\tfrac{1}{4}}</math></small>, <small><math>\sqrt{\tfrac{1}{4}}</math></small>, first from a 16-cell vertex to a 16-cell edge center, then turning 90° to a 16-cell face center, then turning 90° to a 16-cell tetrahedral cell center, then turning 90° to the 16-cell center.

==== Helical construction ====
{{Original research section|date=April 2025}}
[[File:Eight face-bonded tetrahedra.jpg|thumb|A 4-dimensional ring of 8 face-bonded tetrahedra, seen in the [[Boerdijk–Coxeter helix]], bounded by three eight-edge circular paths of different colors, cut and laid out flat in 3-dimensional space. It contains an ''isocline'' axis (not shown), a helical circle of circumference 4𝝅 that twists through all four dimensions and visits all 8 vertices.{{Efn|name=isocline}} The two blue-blue-yellow triangles at either end of the cut ring are the same object.]]
[[File:16-cell 8-ring net4.png|thumb|Net and orthogonal projection]]

A 16-cell can be constructed (three different ways) from two [[Boerdijk–Coxeter helix]]es of eight chained tetrahedra, each bent in the fourth dimension into a ring.{{Sfn|Coxeter|1970|loc=Table 2: Reflexible honeycombs and their groups|p=45|ps=; Honeycomb [3,3,4]<sub>4</sub> is a tiling of the 3-sphere by 2 rings of 8 tetrahedral cells.}}{{Failed verification|date=April 2025|reason=As an initial matter, Coxeter 1970 never uses the symbol [3,3,4]_4. The following discussion assumes that {3,3,4}_4 was meant. Section 11 of Coxeter 1970 describes that {3,3,4}_4 is a honeycomb that is generated from the 16-cell {3,3,4} by identifying antipodal points on the 3-sphere, i.e., it is a honeycomb in elliptic 3-space that is topologically different from a 16-cell. It has half the cells (8), faces (16), edges (12), and vertices (4) as a 16-cell (see Table 2 in Coxeter 1970). Coxeter 1970 does not state anywhere that the 16-cell is composed of two rings of eight tetrahedral cells and it does not follow from the comment in the source immediately before this note.}}{{Sfn|Banchoff|2013}}{{Failed verification|date=April 2025|reason=Banchoff 2013 describes the decomposition of the 8-cell and 24-cell into tori, but neither discusses the decomposition of the 16-cell into chained tetrahedra or its construction therefrom nor anything relating to Boerdijk–Coxeter helices.}} The two circular helixes spiral around each other, nest into each other and pass through each other forming a [[Hopf link]]. The 16 triangle faces can be seen in a 2D net within a [[triangular tiling]], with 6 triangles around every vertex. The purple edges represent the [[Petrie polygon]] of the 16-cell. The eight-cell ring of tetrahedra contains three [[octagram]]s of different colors, eight-edge circular paths that wind twice around the 16-cell on every third vertex of the octagram. The orange and yellow edges are two four-edge halves of one octagram, which join their ends to form a [[Möbius strip]].

Thus the 16-cell can be decomposed into two cell-disjoint circular chains of eight tetrahedrons each, four edges long, one spiraling to the right (clockwise) and the other spiraling to the left (counterclockwise). The left-handed and right-handed cell rings fit together, nesting into each other and entirely filling the 16-cell, even though they are of opposite chirality. This decomposition can be seen in a 4-4 [[duoantiprism]] construction of the 16-cell: {{CDD|node_h|2x|node_h|2x|node_h|2x|node_h}} or {{CDD|node|4|node_h|2x|node_h|4|node}}, [[Schläfli symbol]] {2}⨂{2} or s{2}s{2}, [[Coxeter notation|symmetry]] [4,2<sup>+</sup>,4], order 64.

Three eight-edge paths (of different colors) spiral along each eight-cell ring, making 90° angles at each vertex. (In the Boerdijk–Coxeter helix before it is bent into a ring, the angles in different paths vary, but are not 90°.) Three paths (with three different colors and apparent angles) pass through each vertex. When the helix is bent into a ring, the segments of each eight-edge path (of various lengths) join their ends, forming a Möbius strip eight edges long along its single-sided circumference of 4𝝅, and one edge wide.{{Efn|name=Möbius circle}} The six four-edge halves of the three eight-edge paths each make four 90° angles, but they are ''not'' the six orthogonal great squares: they are open-ended squares, four-edge 360° helices whose open ends are [[antipodal point|antipodal]] vertices. The four edges come from four different great squares, and are mutually orthogonal. Combined end-to-end in pairs of the same [[chirality]], the six four-edge paths make three eight-edge Möbius loops, [[helix|helical]] octagrams. Each octagram is both a [[Petrie polygon]] of the 16-cell, and the helical track along which all eight vertices rotate together, in one of the 16-cell's distinct isoclinic [[#Rotations|rotations]].{{Efn|The 16-cell can be constructed from two cell-disjoint eight-cell rings in three different ways; it has three orientations of its pair of rings. Each orientation "contains" a distinct left-right pair of isoclinic rotations, and also a pair of completely orthogonal great squares (Clifford parallel fibers), so each orientation is a discrete [[Hopf fibration|fibration]] of the 16-cell. Each eight-cell ring contains three axial octagrams which have different orientations (they exchange roles) in the three discrete fibrations and six distinct isoclinic rotations (three left and three right) through the cell rings. Three octagrams (of different colors) can be seen in the illustration of a single cell ring, one in the role of Petrie polygon, one as the right isocline, and one as the left isocline. Because each octagram plays three roles, there are exactly six distinct isoclines in the 16-cell, not 18.|name=only one disjoint pair of eight-cell rings}}

{| class="wikitable" width=610
!colspan=5|Five ways of looking at the same [[Skew polygon|skew]] [[octagram]]{{Efn|All five views are the same orthogonal projection of the 16-cell into the same plane (a circular cross-section of the eight-cell ring cylinder), looking along the central axis of the cut ring cylinder pictured above, from one end of the cylinder. The only difference is which {{radic|2}} edges and {{radic|4}} chords are ''omitted'' for focus. The different colors of {{radic|2}} edges appear to be of different lengths because they are oblique to the viewer at different angles. Vertices are numbered 1 (top) through 8 in counterclockwise order.}}
|-
![[#Rotations|Edge path]]
![[Petrie polygon#The Petrie polygon of regular polychora (4-polytopes)|Petrie polygon]]{{Sfn|Coxeter|1973|pp=292-293|loc=Table I(ii); 24-cell ''h<sub>1</sub>''}}
!16-cell
![[Hopf fibration|Discrete fibration]]
![[#Coordinates|Diameter chords]]
|-
![[Octagram]]<sub>{8/3}</sub>{{Sfn|Coxeter|1973|pp=292-293|loc=Table I(ii); 24-cell ''h<sub>2</sub>''}}
![[Petrie polygon#The Petrie polygon of regular polychora (4-polytopes)|Octagram]]<sub>{8/1}</sub>
![[Coxeter element#Coxeter plane|Coxeter plane]] [[B4 polytope|B<sub>4</sub>]]
![[Octagram#Star polygon compounds|Octagram]]<sub>{8/2}=2{4}</sub>
![[Octagram#Star polygon compounds|Octagram]]<sub>{8/4}=4{2}</sub>
|-
|align=center|[[File:16-cell skew octagram (8-3).png|120px]]
|align=center|[[File:16-cell skew octagram (8).png|120px]]
|align=center|[[File:16-cell skew octagram.png|120px]]
|align=center|[[File:16-cell skew octagram 2(4).png|120px]]
|align=center|[[File:16-cell skew octagram 4(2).png|120px]]
|-
|The eight {{radic|2}} chords of the edge-path of an isocline.{{Efn|name=isocline curve}}
|Skew [[octagon]] of eight {{radic|2}} edges. The 16-cell has 3 of these 8-vertex circuits.
|All 24 {{radic|2}} edges and the four {{radic|4}} orthogonal axes.
|Two completely orthogonal (disjoint) great squares of {{radic|2}} edges.{{Efn|name=Clifford parallel great squares}}
|The four {{radic|4}} chords of an isocline. Every fourth isocline vertex is joined to its antipodal vertex by a 16-cell axis.{{Efn|Each isocline has the eight continuous {{radic|2}} chords of its octagram<sub>{8/3}</sub> edge-path, and also four discontinuous {{radic|4}} diameter chords that connect every ''fourth'' vertex on the octagram but do not connect to each other. Antipodal vertices also have a twisted continuous path of four mutually orthogonal {{radic|2}} edges connecting them. Between antipodal vertices, the isocline curves smoothly around in a helix over the four {{radic|2}} chords of its edge-path, hitting the three intervening vertices. Each {{radic|2}} edge is an edge of a great square, that is completely orthogonal to another great square, in which the {{radic|4}} chord is a diagonal.|name=isocline curve}}
|}
Each eight-edge helix is a [[Skew polygon|skew]] [[octagram]] {8/3} that [[winding number|winds three times]] around the 16-cell and visits every vertex before closing into a loop. Its eight {{radic|2}} edges are chords of an ''isocline'', a helical arc on which the 8 vertices circle during an isoclinic rotation.{{Efn|An isocline is a circle of special kind corresponding to a pair of [[Villarceau circle]]s linked in a [[Möbius loop]]. It curves through four dimensions instead of just two. All ordinary circles have a 2𝝅 circumference, but the 16-cell's isocline is a circle with an 4𝝅 circumference (over eight 90° chords). An isocline is a circle that does not lie in a plane, but to avoid confusion we always refer to it as an ''isocline'' and reserve the term ''circle'' for an ordinary circle in the plane.|name=Möbius circle}} All eight 16-cell vertices are {{radic|2}} apart except for opposite (antipodal) vertices, which are {{radic|4}} apart. A vertex moving on the isocline visits three other vertices that are {{radic|2}} apart before reaching the fourth vertex that is {{radic|4}} away.{{Efn|In the 16-cell, two antipodal vertices are opposite vertices of two face-bonded tetrahedral cells. The two antipodal vertices are connected by (three different) two-edge great circle paths along edges of the tetrahedral cells, by various three-edge paths, and by four-edge paths on isoclines and Petrie polygons. {{Efn|name=Möbius circle}}|name=isocline}}

The eight-cell ring is [[chiral]]: there is a right-handed form which spirals clockwise, and a left-handed form which spirals counterclockwise. The 16-cell contains one of each, so it also contains a left and a right isocline; the isocline is the circular axis around which the eight-cell ring twists. Each isocline visits all eight vertices of the 16-cell.{{Efn|In the 16-cell each ''single'' isocline winds through all 8 vertices: an entire [[Hopf fibration|fibration]] of two completely orthogonal great squares.{{Efn|name=completely orthogonal Clifford parallels are special}} The 5-cell and the 16-cell are the only regular 4-polytopes where each discrete fibration has just one isocline fiber.{{Efn|Except in the 5-cell and 16-cell,{{Efn|name=two special cases}} a pair of left and right isocline circles have disjoint vertices: the left and right isocline helices are non-intersecting parallels but counter-rotating, forming a special kind of double helix which cannot occur in three dimensions (where counter-rotating helices of the same radius must intersect).|name=counter-rotating double helix}}|name=each 16-cell isocline reaches all 8 vertices}} Each eight-cell ring contains half of the 16 cells, but all 8 vertices; the two rings share the vertices, as they nest into each other and fit together. They also share the 24 edges, though left and right octagram helices are different eight-edge paths.{{Efn|The left and right isoclines intersect each other at every vertex. They are different sequences of the same set of 8 vertices. With respect only to the set of 4 vertex pairs which are {{radic|2}} apart, they can be considered to be Clifford parallel. With respect only to the set of 4 vertex pairs which are {{radic|4}} apart, they can be considered to be completely orthogonal.{{Efn|name=completely orthogonal Clifford parallels are special}}}}

Because there are three pairs of completely orthogonal great squares,{{Efn|name=six orthogonal planes of the Cartesian basis}} there are three congruent ways to compose a 16-cell from two eight-cell rings. The 16-cell contains three left-right pairs of eight-cell rings in different orientations, with each cell ring containing its axial isocline.{{Efn|name=only one disjoint pair of eight-cell rings}} Each left-right pair of isoclines is the track of a left-right pair of distinct isoclinic rotations: the rotations in one pair of completely orthogonal invariant planes of rotation.{{Efn|name=Clifford parallel great squares}} At each vertex, there are three great squares and six octagram isoclines that cross at the vertex and share a 16-cell axis chord.{{Efn|This is atypical for isoclinic rotations generally; normally both the left and right isoclines do not occur at the same vertex: there are two disjoint sets of vertices reachable only by the left or right rotation respectively.{{Efn|name=counter-rotating double helix}} The left and right isoclines of the 16-cell form a very special double helix: unusual not just because it is circular, but because its different left and right helices twist around each other through the ''same set'' of antipodal vertices,{{Efn|name=each 16-cell isocline reaches all 8 vertices}} not through the two ''disjoint subsets'' of antipodal vertices, as the isocline pairs do in most isoclinic rotations found in nature.{{Efn|For another example of the left and right isoclines of a rotation visiting the same set of vertices, see the [[5-cell#Geodesics and rotations|characteristic isoclinic rotation of the 5-cell]]. Although in these two special cases left and right isoclines of the same rotation visit the same set of vertices, they still take very different rotational paths because they visit the same vertices in different sequences.|name=two special cases}} Isoclinic rotations in completely orthogonal invariant planes are special.{{Efn|Each great square plane is isoclinic (Clifford parallel) to five other square planes but completely orthogonal to only one of them. Every pair of completely orthogonal planes has Clifford parallel great circles, but not all Clifford parallel great circles are orthogonal. There is also another way in which completely orthogonal planes are in a distinguished category of Clifford parallel planes: they are not [[chiral]]. A pair of isoclinic (Clifford parallel) planes is either a ''left pair'' or a ''right pair'' unless they are separated by two angles of 90° (completely orthogonal planes) or 0° (coincident planes).{{Sfn|Kim|Rote|2016|pp=7-8|loc=§&nbsp;6 Angles between two Planes in 4-Space|ps=; Left and Right Pairs of Isoclinic Planes.}} Most isoclinic planes are brought together only by a left isoclinic rotation or a right isoclinic rotation, respectively. Completely orthogonal planes are special: the pair of planes is both a left and a right pair, so either a left or a right isoclinic rotation will bring them together. Because planes separated by a 90° isoclinic rotation are 180° apart, the plane to the ''left'' and the plane to the ''right'' are the same plane.{{Efn|name=exchange of completely orthogonal planes}}|name=completely orthogonal Clifford parallels are special}} To see ''how'' and ''why'' they are special, visualize two completely orthogonal invariant planes of rotation, each rotating by some rotation angle ''and'' tilting sideways by the same rotation angle into a different plane entirely.{{Efn|name=isoclinic rotation}} ''Only when the rotation angle is 90°,'' that different plane in which the tilting invariant plane lands will be the completely orthogonal invariant plane itself. The destination plane of the rotation ''is'' the completely orthogonal invariant plane. The 90° isoclinic rotation is the only rotation which takes the completely orthogonal invariant planes to each other.{{Efn|name=exchange of completely orthogonal planes}} This reciprocity is the reason both left and right rotations go to the same place.}}