page_title: Business telephone system
section_title: Electromechanical shared-control key system
hierarchical_section_title: Business telephone system / Key telephone system / Electromechanical shared-control key system
caption_attribution_description: An eighteen-button Western Electric Call Director with a rotary dial. Please credit David Jordan when using this photo.
context_page_description: A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system to the private branch exchange.
  A business telephone system differs from an installation of several telephones with multiple central office lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, and that such a system often provides additional features related to call handling. Business telephone systems are often broadly classified into key telephone systems, and private branch exchanges, but many hybrid systems exist.
  A key telephone system was originally distinguished from a private branch exchange in that it did not require an operator or attendant at the switchboard to establish connections between the central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office system in capacity and features.
context_section_description: Before the advent of large-scale integrated circuits, key systems were typically composed of electromechanical components (relays) as were larger telephone switching systems.
  The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System are typical examples and sold for many decades. The 1A family of Western Electric Company (WECo) key telephone units (KTUs) were introduced in the late 1930s and remained in use to 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. Introduced in 1953, 1A1 key systems simplified wiring with a single KTU for both line and station termination, and increased the features available. As the 1A1 systems became commonplace, requirements for intercom features grew. 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. The 6A systems were complex, troublesome and expensive, and never became popular. The advent of 1A2 technology in the 1964 simplified key system set up and maintenance. These continued to be used throughout the 1980s, when the arrival of electronic key systems with their easier installation and greater features signaled the end of electromechanical key systems.
  Two lesser-known 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 produced a family of key telephone equipment, some of it compatible with Western Electric equipment, but it did not gain the widespread use enjoyed by Western Electric equipment.
caption: A typical rotary dial key telephone: the Western Electric eighteen-button Call Director, manufactured from 1958 to the early '80s.
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