The ability to pattern surfaces on a microscopic length scale is of importance for technological applications such as the fabrication of microelectronic circuits and digital storage media. Devices fabricated entirely from polymers are now available, opening up the possibility of adapting polymer processing technologies to fabricate cheap, large-area devices using non-lithographic techniques(1,2)-for example, by exploiting dewetting(3) and phase separation(4-6) in thin films. But the final pattern adopted by the polymer film using such approaches requires a template printed onto the substrate by optical lithography, microcontact printing(4,5) or vapour deposition(3). Here we describe a simple process for patterning surfaces that does not require a template. Our method involves the spinodal dewetting of a polymer surface by a thin polymer film, in which a liquid film breaks up owing to the amplification of thermal fluctuations in film thickness induced by dispersion forces(7-14). A preferred orientation is imposed on the dewetting process simply by rubbing the substrate, and this gives rise to patterns of remarkably well-aligned polymer lines. The width of these lines is well-defined, and is controlled by the magnitude of the dispersion forces at the interface, which in turn can be varied by varying the thickness of the polymer substrate. We expect that further work will make it possible to optimize the degree of order in the final morphology.