An in-bin stapling sorter has a stack of trays moved to positions above and below a sheet entry location at which the trays are spaced apart by cams to receive sheets of paper from an office copier or printer. Sets of sheets are stapled in the trays as the trays are moved together past a stapler which is moved into a stapling position from a non-stapling position. An aligning arm is disposed at one side of the set of trays to engage a side edge of the sheets fed into the trays and sets of sheets in the trays during stapling to align the side edges of the sets of sheets for moving the sheets and sets of sheets against a straight surface at the other side of the trays, thereby providing a neat stapled set.