Abstract
Discrepancies exist in the literature about the formation and atomic structure of intercalated CuO layers in YBa2Cu3O7−δ, the so-called ⅙[031] stacking faults. In this paper the fault structures are re-analysed depending OR whether the additional CuO layer has expanded into a tetragonal matrix or an orthorhombic matrix. Of particular interest are the interaction features of such a fault with mirror twin boundaries.