# Face diagonal

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In geometry, a **face diagonal** of a polyhedron is a diagonal on one of the faces, in contrast to a space diagonal passing through the interior of the polyhedron.^{[1]}

A cuboid has twelve face diagonals (and four space diagonals);^{[2]} the cuboid's face diagonals can have up to three different lengths (the space diagonals all have the same length). A regular dodecahedron has 60 face diagonals (and 100 space diagonals).^{[3]}

The face diagonal of a cube with side length *a* is .^{[4]}