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The unit cell and space group of orthorhombic NaV6O11 (low-temperature form) are found by an X-ray single-crystal diffraction study at 300 K (hexagonal, P63/mmc), 100 K (hexagonal, P63mc) and 20 K (orthorhombic). The orthorhombic form (o) shows no superstructure and its unit cell is related to the hexagonal one (h): ao ≃ ah + bh, ao ≃ −ah + bh and co ≃ ch. Bijvoet-pair examination confirms that it crystallizes in the noncentrosymmetric space group Cmc21. The hexagonal (P63mc)-orthorhombic (Cmc21) structural phase transition is proved to be of second order.