Abstract: A high speed machine that inserts about twelve thousand sheets or stacks of sheets of paper per hour into the same number of envelopes. A first table delivers an elongate horizontal queue of thousands of envelopes to an envelope hopper. A second table, parallel to the first table, includes a plurality of independently-controllable sheet feeders and delivers individual groups of stacked sheets to an envelope insertion station that is transversely spaced apart from the envelope hopper. A third table is disposed transversely to the first two tables and includes the envelope insertion station. Clamps carried by an elongate sprocket chain mounted in the third table sequentially remove envelopes from the bottom of the envelope hopper and pull them past an envelope flap-opening structure to the envelope insertion station where the envelopes are opened by a puff of air.