The 1A2 Key System is an analog multiline key telephone system used in small businesses and homes. Unlike more modern multiline systems, every telephone line serving a particular phone is wired into that phone, and electromechanical switches (the "keys") select a line to be used. A 1A2 system consists of a central control unit, known as a KSU (Key Service Unit), phones, accessories, and cable. The most common 1A2 phone has six buttons. One button is for Hold, and the other five could be used for a mix of lines, intercom and features. It uses a 25-pair (50-wire) cable, but not all of the wires were connected. There are many variations of the common six-button phone, with modifications including a headset jack, amplified handset, and a key that can rotate to control a function.