Why would the builder of my recently purchased, early '80s house put down 7/16" plywood over the floor joists, then build the partition walls on top of it, and THEN put down 3/4" plywood as underlayment BETWEEN the partition walls? I would have most assuredly put the 3/4 down as the subfloor. In the kitchen he glued linoleum directly to the top of the 3/4" ply, so I have to cut it out in order to lay a new floor. In a way, it's a blessing because it gives me an excuse to install 23/32" AdvanTech over the tissue paper subfloor and to be sure it's fastened properly. In the rest of the house, where he installed carpeting, he used 3/4" particle board as the underlayment, and that gave us a structure in which 80% of the floor structure consisted of 7/16" plywood and 3/4" particle board. Again, to my good fortune, they had small pets that ruined the particle board, and I could tear it all out and install 23/32" AdvanTech before laying 3/4" solid oak planks. Can you imagine the difference underfoot?