Periodic micro- and nanostructures ( gratings) have many significant applications in electronic, optical, magnetic, chemical and biological devices and materials. Traditional methods for fabricating gratings by writing with electrons(1), ions(2) or a mechanical tip are limited to very small areas and suffer from extremely low throughput. Interference lithography can achieve relatively large fabrication areas, but has a low yield for small-period gratings(3,4). Photolithography(5), nanoimprint lithography(6,7), soft lithography(8,9) and lithographically induced self-construction(10) all require a prefabricated mask, and although electrohydrodynamic instabilities(11) can self-produce periodic dots without a mask, gratings remain challenging. Here, we report a new low-cost maskless method to self-generate nano- and microgratings from an initially featureless polymer thin film sandwiched between two relatively rigid flat plates. By simply prising apart the plates, the film fractures into two complementary sets of nonsymmetrical gratings, one on each plate, of the same period. The grating period is always four times the thickness of the glassy film, regardless of its molecular weight and chemical composition. Periods from 120 nm to 200 mu m have been demonstrated across areas as large as two square centimetres.