# Edge (geometry)

__: Line segment joining two adjacent vertices in a polygon or polytope__

**Short description**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.

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 **polygon side**. In a polyhedron or more generally a polytope, an edge is a line segment where two faces (or polyhedron sides) meet.^{[3]} A segment joining two vertices while passing through the interior or exterior is not an edge but instead is called a diagonal.

## Relation to edges in graphs

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

Any convex polyhedron's surface has Euler characteristic

- [math]\displaystyle{ V - E + F = 2, }[/math]

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

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

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

## References

- ↑ Ziegler, Günter M. (1995),
*Lectures on Polytopes*, Graduate Texts in Mathematics,**152**, Springer, Definition 2.1, p. 51, ISBN 9780387943657, https://books.google.com/books?id=xd25TXSSUcgC&pg=PA51. - ↑ Weisstein, Eric W. "Polygon Edge". From Wolfram MathWorld.
- ↑ Weisstein, Eric W. "Polytope Edge". From Wolfram MathWorld.
- ↑ Senechal, Marjorie (2013),
*Shaping Space: Exploring Polyhedra in Nature, Art, and the Geometrical Imagination*, Springer, p. 81, ISBN 9780387927145, https://books.google.com/books?id=kZtCAAAAQBAJ&pg=PA81. - ↑ Gorini, Catherine A., ed. (2000), "Bridges between geometry and graph theory",
*Geometry at work*, MAA Notes,**53**, Washington, DC: Math. Assoc. America, pp. 174–194. 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, http://projecteuclid.org/euclid.pjm/1103037323. - ↑ Wenninger, Magnus J. (1974),
*Polyhedron Models*, Cambridge University Press, p. 1, ISBN 9780521098595, https://books.google.com/books?id=N8lX2T-4njIC&pg=PA1. - ↑ "Constructing higher-dimensional convex hulls at logarithmic cost per face",
*Proceedings of the Eighteenth Annual ACM Symposium on Theory of Computing (STOC '86)*, 1986, pp. 404–413, doi:10.1145/12130.12172.

## External links

- Weisstein, Eric W.. "Polygonal edge". http://mathworld.wolfram.com/PolygonEdge.html.
- Weisstein, Eric W.. "Polyhedral edge". http://mathworld.wolfram.com/PolyhedronEdge.html.

Original source: https://en.wikipedia.org/wiki/Edge (geometry).
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