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. Hybrid key telephone system Into the 21st century, the distinction between key systems and PBX systems has become increasingly blurred. 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 SIP, ISDN, analog handsets (in addition to its own proprietary handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. Their support for both analog and digital signaling, and of some PBX functionality gives rise to the hybrid designation. A hybrid system typically has some call appearance buttons that directly correspond to individual lines and/or stations, but may also support direct dialing to extensions or outside lines without selecting a line appearance. The modern key system is usually fully digital, although analog variants persist and some systems implement VOIP services. Effectively, the aspects that distinguish a PBX from a hybrid key system are the amount, scope and complexity of the features and facilities offered. Private branch exchange A PBX is a telephone exchange or switching system that serves a private organization and permits sharing of central office trunks between internally installed telephones, and provides intercommunication between those internal telephones within the organization without the use of external lines. The central office lines provide connections to the public switched telephone network (PSTN) and the concentration aspect of a PBX permits the shared use of these lines between all stations in the organization. Its intercommunication ability allows two or more stations to directly connect while not using the public switched telephone network. This method reduces the number of lines needed from the organization to the public switched telephone network. Each device connected to the PBX, such as a telephone, a fax machine, or a computer modem, is referred to as an extension and has a designated extension telephone number that may or may not be mapped automatically to the numbering plan of