Although YBa2Cu3O7 is a well-defined chemical compound with a specific structure and stoichiometry, materials with fewer than seven oxygen atoms per formula unit are non-stoichiometric compounds. The structure of these materials depends on the oxygen content. This non-stoichiometry is denoted by the x in the chemical formula YBa2Cu3O7−"x". When "x" = 1, the O(1) sites in the Cu(1) layer are vacant and the structure is tetragonal. The tetragonal form of YBCO is insulating and does not superconduct. Increasing the oxygen content slightly causes more of the O(1) sites to become occupied. For "x" < 0.65, Cu-O chains along the "b" axis of the crystal are formed. Elongation of the "b" axis changes the structure to orthorhombic, with lattice parameters of "a" = 3.82, "b" = 3.89, and "c" = 11.68 Å. Optimum superconducting properties occur when "x" ~ 0.07, i.e., almost all of the O(1) sites are occupied, with few vacancies.