# Face (geometry)

In solid geometry, a **face** is a flat (planar) surface that forms part of the boundary of a solid object;^{[1]} a three-dimensional solid bounded exclusively by flat faces is a polyhedron.

In more technical treatments of the geometry of polyhedra and higher-dimensional polytopes, the term is also used to mean an element of any dimension of a more general polytope (in any number of dimensions).^{[2]}

## Contents

## Polygonal face[edit]

In elementary geometry, a **face** is a polygon on the boundary of a polyhedron.^{[2]}^{[3]} Other names for a polygonal face include **side** of a polyhedron, and **tile** of a Euclidean plane tessellation.

For example, any of the six squares that bound a cube is a face of the cube. Sometimes "face" is also used to refer to the 2-dimensional features of a 4-polytope. With this meaning, the 4-dimensional tesseract has 24 square faces, each sharing two of 8 cubic cells.

Polyhedron | Star polyhedron | Euclidean tiling | Hyperbolic tiling | 4-polytope |
---|---|---|---|---|

{4,3} | {5/2,5} | {4,4} | {4,5} | {4,3,3} |

The cube has 3 square faces per vertex. |
The small stellated dodecahedron has 5 pentagrammic faces per vertex. |
The square tiling in the Euclidean plane has 4 square faces per vertex. |
The order-5 square tiling has 5 square faces per vertex. |
The tesseract has 3 square faces per edge. |

Some other polygons, which are not faces, are also important for polyhedra and tessellations. These include Petrie polygons, vertex figures and facets (flat polygons formed by coplanar vertices which do not lie in the same face of the polyhedron).

### Number of polygonal faces of a polyhedron[edit]

Any convex polyhedron's surface has Euler characteristic

where *V* is the number of vertices, *E* is the number of edges, and *F* is the number of faces. This equation is known as Euler's polyhedron formula. Thus the number of faces is 2 more than the excess of the number of edges over the number of vertices. For example, a cube has 12 edges and 8 vertices, and hence 6 faces.

*k*-face[edit]

In higher-dimensional geometry the faces of a polytope are features of all dimensions.^{[2]}^{[4]}^{[5]} A face of dimension *k* is called a *k*-face. For example, the polygonal faces of an ordinary polyhedron are 2-faces. In set theory, the set of faces of a polytope includes the polytope itself and the empty set where the empty set is for consistency given a "dimension" of −1. For any *n*-polytope (*n*-dimensional polytope), −1 ≤ *k* ≤ *n*.

For example, with this meaning, the faces of a cube include the empty set, its vertices (0-faces), edges (1-faces) and squares (2-faces), and the cube itself (3-face).

All of the following are the **faces** of a 4-dimensional polytope:

- 4-face – the 4-dimensional 4-polytope itself
- 3-faces – 3-dimensional cells (polyhedral faces)
- 2-faces – 2-dimensional faces (polygonal faces)
- 1-faces – 1-dimensional edges
- 0-faces – 0-dimensional vertices
- the empty set, which has dimension −1

In some areas of mathematics, such as polyhedral combinatorics, a polytope is by definition convex. Formally, a face of a polytope *P* is the intersection of *P* with any closed halfspace whose boundary is disjoint from the interior of *P*.^{[6]} From this definition it follows that the set of faces of a polytope includes the polytope itself and the empty set.^{[4]}^{[5]}

In other areas of mathematics, such as the theories of abstract polytopes and star polytopes, the requirement for convexity is relaxed. Abstract theory still requires that the set of faces include the polytope itself and the empty set.

### Cell or 3-face[edit]

A **cell** is a polyhedral element (**3-face**) of a 4-dimensional polytope or 3-dimensional tessellation, or higher. Cells are facets for 4-polytopes and 3-honeycombs.

Examples:

4-polytopes | 3-honeycombs | ||
---|---|---|---|

{4,3,3} | {5,3,3} | {4,3,4} | {5,3,4} |

The tesseract has 3 cubic cells (3-faces) per edge. |
The 120-cell has 3 dodecahedral cells (3-faces) per edge. |
The cubic honeycomb fills Euclidean 3-space with cubes, with 4 cells (3-faces) per edge. |
The order-4 dodecahedral honeycomb fills 3-dimensional hyperbolic space with dodecahedra, 4 cells (3-faces) per edge. |

### Facet or (*n*-1)-face[edit]

In higher-dimensional geometry, the **facets** of a *n*-polytope are the (*n*-1)-faces of dimension one less than the polytope itself.^{[7]} A polytope is bounded by its facets.

For example:

- The facets of a line segment are its 0-faces or vertices.
- The facets of a polygon are its 1-faces or edges.
- The facets of a polyhedron or plane tiling are its 2-faces.
- The facets of a 4D polytope or 3-honeycomb are its 3-faces.
- The facets of a 5D polytope or 4-honeycomb are its 4-faces.

### Ridge or (*n*-2)-face[edit]

In related terminology, a (*n* − 2)-*face* of an *n*-polytope is called a **ridge** (also **subfacet**).^{[8]} A ridge is seen as the boundary between exactly two facets of a polytope or honeycomb.

For example:

- The ridges of a 2D polygon or 1D tiling are its 0-faces or vertices.
- The ridges of a 3D polyhedron or plane tiling are its 1-faces or edges.
- The ridges of a 4D polytope or 3-honeycomb are its 2-faces or simply
**faces**. - The ridges of a 5D polytope or 4-honeycomb are its 3-faces or cells.

### Peak or (*n*-3)-face[edit]

A (*n* − 3)-*face* of an *n*-polytope is called a **peak**. A peak contain a rotational axis of facets and ridges in a regular polytope or honeycomb.

For example:

- The peaks of a 3D polyhedron or plane tiling are its 0-faces or vertices.
- The peaks of a 4D polytope or 3-honeycomb are its 1-faces or edges.
- The peaks of a 5D polytope or 4-honeycomb are its 2-faces or simply
**faces**.

## See also[edit]

## References[edit]

**^***Merriam-Webster's Collegiate Dictionary*(Eleventh ed.). Springfield, MA: Merriam-Webster. 2004.- ^
^{a}^{b}^{c}Matoušek, Jiří (2002),*Lectures in Discrete Geometry*, Graduate Texts in Mathematics,**212**, Springer, 5.3 Faces of a Convex Polytope, p. 86. **^**Cromwell, Peter R. (1999),*Polyhedra*, Cambridge University Press, p. 13.- ^
^{a}^{b}Grünbaum, Branko (2003),*Convex Polytopes*, Graduate Texts in Mathematics,**221**(2nd ed.), Springer, p. 17. - ^
^{a}^{b}Ziegler, Günter M. (1995),*Lectures on Polytopes*, Graduate Texts in Mathematics,**152**, Springer, Definition 2.1, p. 51. **^**Matoušek (2002) and Ziegler (1995) use a slightly different but equivalent definition, which amounts to intersecting*P*with either a hyperplane disjoint from the interior of*P*or the whole space.**^**Matoušek (2002), p. 87; Grünbaum (2003), p. 27; Ziegler (1995), p. 17.**^**Matoušek (2002), p. 87; Ziegler (1995), p. 71.