A key system or key telephone system is a multiline telephone system typically used in small office environments.
Key systems are noted for their expandability and having individual line selection buttons for each connected phone line, however some features of a private branch exchange such as dialable intercoms may also commonly be present.
Key systems can be built using three principal architectures:
The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System were entirely typical and sold for many decades. The 1A family of Western Electric Company (WECo) key telephone units (KTUs) were in use in the 1950s. 1A equipment was primitive and required at least two KTUs per line; one for line termination and one for station (telephone instrument) termination. The telephone instrument commonly used by 1A systems was the WECo 300-series telephone. In the 1960s, 1A1 key systems simplified wiring with a single KTU for both line and station termination, and increased the features available to the users. As the 1A1 systems became commonplace, requirements for intercom features increased. The original intercom KTUs, WECo Model 207, were wired for a single talk link, that is, a single conversation on the intercom at a time. The WECo 6A dial intercom system provided two talk links and was often installed as the dial intercom in a 1A1 or 1A2 key system. Unfortunately, the 6A systems were very complex and prone to problems. They were also very expensive and never came much into favor. The advent of 1A2 technology in the 1970s tremendously simplified key system set up and maintenance. These continued to be used throughout the 1980s, though the arrival of electronic key systems with their easier installation and greater features signaled the end of all electromechanical key systems.
Two obscure key systems were used at airports for air traffic control communications, the 102 and 302 key systems. These were uniquely designed for communications between the air traffic control tower and radar approach control (RAPCON) or ground control approach (GCA), and included radio line connections.
Automatic Electric Company also sold a family of key telephone equipment, but it never gained the widespread use enjoyed by Western Electric equipment.
With the advent of LSI ICs, the same architecture could be implemented much less expensively than was possible using relays. In addition, it was possible to eliminate the many-wire cabling and replace it with much simpler cable similar to (or even identical to) that used by non-key systems. One of the most recognized such systems is the AT&T Merlin.
Additionally, these more modern systems allowed a vast set of features including:
Features could be added or modified simply using software, allowing easy customization of these systems. The stations were easier to maintain than the previous electromechanical key systems, as they used efficient LEDs instead of incandescent light bulbs for line status indication.
LSI also allowed smaller systems to distribute the control (and features) into individual telephone sets that don't require any single shared control unit. Generally, these systems are used with a relatively few telephone sets and it is often more difficult to keep the feature set (such as speed-dialing numbers) in synchrony between the various sets.
The line between the largest key systems and full PBX systems is blurred. In the 1A2 days, the line was clear: 1A2 systems did not allow the sharing of anonymous "trunk" lines and PBX systems did. Modern key systems blur this distinction by often allowing this feature.
Into the 21st century, the distinction between key systems and PBX has become increasingly confusing. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. The modern key system now supports ISDN, analog handsets (in addition to its own dedicated handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. The fact that they support both analog and digital signalling types gives rise to the "Hybrid" designation.
The modern key system is usually fully digital (although analog variants persist) and some systems embrace VOIP. Indeed, key systems now can be considered to have left their humble roots and become small PBXes. Effectively, the aspects that separate a PBX from a key system are the amount, scope and complexity of the features and facilities offered.