Thin films of symmetric polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) with a bulk lamellar period of L-o were equilibrated on chemically nanopatterned surfaces that had alternating PS and PMMA preferential wetting stripes. The chemical patterns had a period of L-S, and the width of the PMMA wetting stripes was W. We identified a set of four morphologies, two of which are fully threedimensional. On chemical patterns with L-S = 2L(o), as W/L-0 increased from 0 to 1, block copolymers formed morphologies including bulk-like structures, nominally, parallel lamellae and vertical lamellae, and two bulk-deviate complex morphologies. On chemical patterns with W/L-0 = 0.5, as the commensurability factor, delta = L-S/L-0, increased from 0.87 to 3.05, block copolymers formed complex three-dimensional structures. At the substrate, the block copolymer had a wetting layer reflecting the chemical pattern. At the free surface, the block copolymer formed lamellar-like structures with period L-0 along the direction of the underlying chemically patterned stripes when delta was close to an integer, and fingerprint-like structures when delta was not close to an integer. Morphologies obtained by numerical simulations of a coarse grained model of block copolymers compare favorably with our experimental results and help explain the origin of the observed behavior.