Among hexagonal perovskites, the B-cation deficient perovskite-related compounds AnBn-1 O 3 n present two kinds of structural types depending on the stacking sequences of their AO 3 layers. All these structures derive from the perovskite, which is periodically disturbed by planar defects. The structures with ( hhc… c)-type sequences, present successive perovskite blocks shifted from one another by a 1/3<01 1̄0> H vector, while ( hc… c)-type sequences show twin plane boundaries. The non-stoichiometry of these compounds is in all cases closely associated with the distribution of these planar defects: shift planes or twin boundaries. In the hexagonal perovskites of the AnBn-1 O 3 n series belonging to the Ba 5Nb 4O 15-BaTiO 3 system, the twin type structures is favored by high values of n (number of octahedra layers within a perovskite blocks) and t (Goldschmidt tolerance factor). The stability of the "shift" type structure surely comes from the preservation of a bcc cationic sub-lattice. However, this structural type results in the occurrence of a long-range order of vacant octahedra layers. The stability of the "twin" type structure is related to the drastic decrease of the periodicity of vacancies along the c-axis (every n/2 octahedra layers) but needs a very expanded three dimensional BO 6 octahedra sub-lattice (high value of t) and a specific order of the B-cations in the vicinity of the twin planes.