The phat PS2 and slim PS2 have slight different form factors as it relates to the depth of the controller and memory card ports. The original official multitap for the phat PS2 had a solid piece that inserted into the controller and memory card port on the phat PS2, which wouldn't fit quite right in the slim PS2. Later official mutitaps had a split wire with separate connections for the controller and memory card port, so they could be used in either the phat or slim PS2.