Negative tetrahedron

“The tetrahedron is a form composed of four equilateral triangular faces, each of which intersects all of the crystallographic axes at equal lengths. It can be considered as derived from the octahedron of the Normal Class by the omission of the alternate faces and the extension of the others. If the other four faces of the octahedron had been extended, the tetrahedron resulting would have had a different orientation, known as the negative tetrahedron.” — Ford, 1912

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