The self-assembly of block-copolymer thin films in periodic nanostructures has received considerable attention during the last decade due to their potential applications in nanofabrication and nanolithography. We followed the morphologies developed in thin films of a cylinder-forming diblock copolymer polystyrene-b-poly (methylmethacrylate)((PS-b-PMMA), PS 46.1 kg mol− 1, PMMA 21.0 kg mol− 1, lattice spacing L 0= 36 nm), as a function of the film thickness (t), analyzing the effect of thickness commensurability on domain orientation in respect to the substrate. The study was circumscribed to the unexplored range of thickness below L 0. Two thickness windows with perpendicular orientation of the PMMA domains were identified: a well-known window at t~ L 0 and a new window at t~ L 0/2. A half-parallel cylinder morphology was observed for with a progressive change in morphology when thickness increases …