The self-assembly of block copolymers thin films laterally confined within square geometries that are incompatible with the bulk packing symmetry of the block copolymer microdomains is investigated. The lateral confinement is provided by chemical patterns made by oxidation nanolithography of octadecyltrichlorosilane-coated silicon surfaces. We find that the size and shape of the confinement affect the order of the block copolymer microdomains and their packing symmetries. Specifically, if the size of the square pattern is smaller than the characteristic grain size the hexagonally packed microdomains form a single-crystal oriented along the edges of the pattern and with “edge-boundaries” located preferentially at the perimeter of the pattern. If the size of the pattern is comparable with the natural bulk period of the copolymer, the packing symmetry changes from hexagonal to square. In this case the ordering induced by the pattern edges becomes dominant allowing the square lattice to be more stable than the hexagonal one.