# Edge (geometry)

Three edges AB, BC, and CA, each between two vertices of a triangle. |
A polygon is bounded by edges; this square has 4 edges. |

Every edge is shared by two faces in a polyhedron, like this cube. |
Every edge is shared by three or more faces in a 4-polytope, as seen in this projection of a tesseract. |

*For edge in graph theory, see Edge (graph theory)*

In geometry, an **edge** is a particular type of line segment joining two vertices in a polygon, polyhedron, or higher-dimensional polytope.^{[1]} In a polygon, an edge is a line segment on the boundary,^{[2]} and is often called a **side**. In a polyhedron or more generally a polytope, an edge is a line segment where two faces meet.^{[3]} A segment joining two vertices while passing through the interior or exterior is not an edge but instead is called a diagonal.

## Contents

## Relation to edges in graphs[edit]

In graph theory, an edge is an abstract object connecting two graph vertices, unlike polygon and polyhedron edges which have a concrete geometric representation as a line segment.
However, any polyhedron can be represented by its skeleton or edge-skeleton, a graph whose vertices are the geometric vertices of the polyhedron and whose edges correspond to the geometric edges.^{[4]} Conversely, the graphs that are skeletons of three-dimensional polyhedra can be characterized by Steinitz's theorem as being exactly the 3-vertex-connected planar graphs.^{[5]}

## Number of edges in 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 edges is 2 less than the sum of the numbers of vertices and faces. For example, a cube has 8 vertices and 6 faces, and hence 12 edges.

## Incidences with other faces[edit]

In a polygon, two edges meet at each vertex; more generally, by Balinski's theorem, at least *d* edges meet at every vertex of a *d*-dimensional convex polytope.^{[6]}
Similarly, in a polyhedron, exactly two two-dimensional faces meet at every edge,^{[7]} while in higher dimensional polytopes three or more two-dimensional faces meet at every edge.

## Alternative terminology[edit]

In the theory of high-dimensional convex polytopes, a facet or *side* of a *d*-dimensional polytope is one of its (*d* − 1)-dimensional features, a ridge is a (*d* − 2)-dimensional feature, and a peak is a (*d* − 3)-dimensional feature. Thus, the edges of a polygon are its facets, the edges of a 3-dimensional convex polyhedron are its ridges, and the edges of a 4-dimensional polytope are its peaks.^{[8]}

## See also[edit]

## References[edit]

**^**Ziegler, Günter M. (1995),*Lectures on Polytopes*, Graduate Texts in Mathematics,**152**, Springer, Definition 2.1, p. 51.**^**Weisstein, Eric W. "Polygon Edge." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/PolygonEdge.html**^**Weisstein, Eric W. "Polytope Edge." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/PolytopeEdge.html**^**Senechal, Marjorie (2013),*Shaping Space: Exploring Polyhedra in Nature, Art, and the Geometrical Imagination*, Springer, p. 81, ISBN 9780387927145.**^**Pisanski, Tomaž; Randić, Milan (2000), "Bridges between geometry and graph theory", in Gorini, Catherine A. (ed.),*Geometry at work*, MAA Notes,**53**, Washington, DC: Math. Assoc. America, pp. 174–194, MR 1782654. See in particular Theorem 3, p. 176.**^**Balinski, M. L. (1961), "On the graph structure of convex polyhedra in*n*-space",*Pacific Journal of Mathematics*,**11**(2): 431–434, doi:10.2140/pjm.1961.11.431, MR 0126765.**^**Wenninger, Magnus J. (1974),*Polyhedron Models*, Cambridge University Press, p. 1, ISBN 9780521098595.**^**Seidel, Raimund (1986), "Constructing higher-dimensional convex hulls at logarithmic cost per face",*Proceedings of the Eighteenth Annual ACM Symposium on Theory of Computing (STOC '86)*, pp. 404–413, doi:10.1145/12130.12172.