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For a polygon, a diagonal is a line segment connecting two nonadjacent vertices or two vertices that are not next to each other.

Below, we show all the diagonals for a rectangle and a pentagon. There are two diagonals in the rectangle and five diagonals in the pentagon.

For a polyhedron or solid figure, a diagonal is a line segment connecting two vertices that are in different faces.

The following image shows a diagonal for the rectangular prism.