Key Systems
Key was a Bell System term of art for a manually operated switch, such as the line-buttons on the phones associated with such systems.
Key systems are primarily defined by their individual line selection buttons for each connected phone line, a feature shared with hybrid systems. New installations of true "key" systems have become less common, as hybrid systems and private branch exchanges of comparable size now have similar costs and greater functionality.
Key systems can be built using three principal architectures: electromechanical shared-control, electronic shared-control, or independent keysets.
Read more about this topic: Business Telephone System
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