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A tetragonal $A{B}_2$ crystal structure with sixfold cation and threefold anion coordination numbers. A common fractional-coordinate description has A at $(0,0,0)$ and $(1/2,1/2,1/2)$, and B at $\pm (u,u,0)$ and $\pm (1/2+u,1/2-u,1/2)$, interpreted modulo integer lattice translations. The cation coordination is a distorted octahedron and the anion coordination is a planar triangle. Titanium dioxide in its rutile polymorph is an example.