That was back in the day when you could buy 3 8,10,12, or 15w6's and wire them up to get a 4 ohm mono load. Usually three subs were the ideal width to fill the length of a trunk. JL Audio also had those triangular custom fiberglass mounting plates you could buy. The 4 ohm mono load is perfect for amps, but it sucks when you try to run 1 or 2 subs. With one sub you could run the 3 ohm mono(1.5 ohms stereo) load, but some amps couldn't take that impedence long. They are GREAT subs though.