A novel type of dislocation in YBa2Cu3O7, has been identified and analysed by high resolution electron microscopy. Non-stoichiometry is accommodated by means of Frank type sessile dislocations. In the double CuO layers, resulting from the insertion of an extra CuO layer, sometimes a 90° reorientation of the chains occurs along the layer plane. This orientation change of the double chain can be described as being caused by the propagation of a dislocation with a 1/2 [110] Burgers vector and a (001) glide plane located between the two CuO layers, accompanied by reshuffling of part of the oxygen ions.