The term ‘key’ derives from an original Bell System term for a manually operated switch, such as the line-button on early telephones. Modern KSU phone systems have a central hub (the key system unit) that provides functionality, such as internal calling, that non-key phone systems can’t offer.
A variation of the KSU phone system is the KSU-less system. This offers generally similar functionality but without some of the more specialised (and expensive) technology of the pure KSU system. Cheap and simple to set-up, KSU-less systems are generally only viable for up to 10 users.
Nowadays, the distinction between he largest key systems and corporate-level PABX systems is increasingly blurred, with the main differentiators being the quantity, scope and sophistication of available functionality. Modern hybrid key systems are virtually small PBXs, fully digital systems that, in some cases, even embrace the latest VoIP technology.