The crystal structures of the alkaline-earth ruthenates Ba5Ru2O9(O-2), Sr4Ru2O9 and Ba4Ru3O10, belong to three different series. They may be described both in terms of an octahedral framework and by the stacking of perovskite-like layers. Taking into account geometrical, structural and crystal chemistry considerations, the relationships between family members in the three series and the principles governing the crystallographic features of hypothetical members are discussed.