The material placed on top of the floor joists may be plywood, Oriented Strand Board (OSB) or particle board. In any case the joists have glue spread on them and then the 4' x 8' panels of sub flooring are nailed in place.

At the time the Sub floor goes down the bottom of the home has been assembled and is resting on its frame and wheels, but has nothing to force it to be square or straight. Since the floor panels are known to be square the house itself is squared up by forcing the joists and side edges to line up with the floor panels. This involves sledge hammers and come-along's and can make for some ugly edges. However, if all else fails and there is still some overhang, a router will fix that :). The floor panels are nailed to the joists and edges with powerful air guns that will drive 4" or 6" nails at a rate of one or more per second. Needless to say, not all nails get driven straight or into the joists. In addition, the depth to which the nail is driven depends on the setting of the air gun. To much power and the nail is driven well below the surface, to little power and the head sticks up. This can make for interesting problems for the carpet and vinyl crews