<Tr> <Td> Every edge is shared by two faces in a polyhedron, like this cube . </Td> <Td> Every edge is shared by three or more faces in a 4 - polytope, as seen in this projection of a tesseract . </Td> </Tr> <Dl> <Dd> For edge in graph theory, see Edge (graph theory) </Dd> </Dl> <Dd> For edge in graph theory, see Edge (graph theory) </Dd> <P> In geometry, an edge is a particular type of line segment joining two vertices in a polygon, polyhedron, or higher - dimensional polytope . In a polygon, an edge is a line segment on the boundary, and is often called a side . In a polyhedron or more generally a polytope, an edge is a line segment where two faces meet . A segment joining two vertices while passing through the interior or exterior is not an edge but instead is called a diagonal . </P>

The edges of a space are often defined by what