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In page Polyhedron:

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When segment lines connect two vertices that are not in the same face, they form the diagonal lines.[2] However, not all polyhedra have diagonal lines, as in the family of pyramids,[citation needed] Schönhardt polyhedron in which three diagonal lines lies entirely outside of it, and Császár polyhedron has no diagonal lines (rather, every pair of vertices is connected by an edge).[3]