Brass Light Socket - Plated - 3-Terminal - 2 Circuit - Turn Knob - Medium Base Socket - 1/8 IPS - PLT 90-421 Medium Base Complete Socket. Two circuit system controls another socket wired onto the third terminal. Aluminum Shell Incandescent Lampholder bright gold finish 1/8 IPS Tapped Bushing on bottom cap Not the same as a 3-way socket, this socket is for controlling 2 light bulbs with one socket.