Abstract: Detailed analysis of the crystal structures of minerals and inorganic compounds containing oxocentred [OCu₄] tetrahedra reveals the following principle for relationships between oxocentred tetrahedra and tetrahedral oxyanions (Tⁿ⁺O₄)⁻⁽⁸⁻ⁿ⁾ with T = S⁶⁺ or V⁵⁺, or triangular pyramidal oxyanions (SeO₃)²⁻ (which may be considered as (SeO₃E)²⁻ tetrahedra assuming the lone pair E of selenium to be a fourth ligand): (Tⁿ⁺O₄)⁻⁽⁸⁻ⁿ⁾ tetrahedra are ‘attached’ to the oxocentred tetrahedra so that their grounds (O-O-O and Cu-Cu-Cu) are parallel to each other and the mutual orientation is regular (O corners are exactly under Cu corners). We propose to describe these relationships as ‘face-to-face’, meaning that the oxocentred and cation-centred tetrahedra ‘meet’ along their ‘whole’ triangular faces. Geometric characteristics of ‘face-to-face’ relationships are discussed.