Control over the orientation of a symmetric block copolymer lamellar morphology using a striped, chemically heterogeneous surface has been established. Films of symmetric diblock copolymers of poly(styrene-bloch-methyl methacrylate) solution cast onto silicon oxide substrates striped with periodic, 20 nm wide gold lines show lamellar microdomain orientation perpendicular to the substrate plane and parallel to the striping. Commensurability of the block copolymer and substrate stripe period is found to be essential to produce such a surface-directed morphology. A mismatch in length scale of only +/-10% is sufficient to lose control over the microdomain orientation. This window of commensurability for thick films is much smaller than that found previously for ultrathin films. Consequently, more stringent conditions are met in unconfined films to direct the morphology with the surface. The distance over which the orientation of the microdomains persists is found to depend on the ordering kinetics, scaling with copolymer molecular weight, and is independent of the microdomain orientation at the surface.