Abstract: A method is provided for controlling an electron beam pattern generator which writes a pattern on a substrate with an electron beam. A pattern store holds the defining coordinates of a set of trapezia 9, 13, 14, 15, 16 which together constitute the pattern. The set of trapezia in the store are divided into groups written in turn on the substrate, successive groups containing successively coarser 13, 14, 15, 16 to finer 9 pattern details. The groups are preceded in the pattern store by respective data words which instruct a trapezium generator to produce scanning steps of successive groups at successively higher to lower frequencies respectively. Coarse detail is thereby written quickly and fine detail receives the correct electron exposure.