Abstract: A thermal series type printing head is controlled on-the-fly, during a succession of cycles of a duration at most equal to the time for printing the points, so as to be heated independently during the first half and during the second half of each cycle. Thus an offset dot can be printed of half a length of a a normal dot, or an extended dot, one and half times as long as the normal dot, for improving the definition of printing without reducing the writing speed. The invention applies to printing systems, particularly for printers connected to word processing devices.