A system for printing envelopes which includes a laser printer (20) operating under control of a microcomputer (40). The printer (20) includes a pair of parallel envelope paths for printing pairs of envelopes simultaneously, and also has a capability to print single envelopes (E) as they are transported along the center line of the printer (20). The microcomputer controls the printer (20) to print sequences of addresses on the envelopes (E). When envelope are printed in pairs the the microcomputer forms subsequences of addresses which are printed on groups of envelopes so that the groups can be concatenated into larger groups with addresses in sequence. Movement through the printer (20) is controlled by pairs of sensor which are "anded" and symmetrically spaced around the center line so that the sensors can control either a pair of envelopes or a single envelope transported along the center line of the printer.