A Private Branch eXchange (also called PBX or Private Business eXchange) is a telephone exchange that is owned by a private business, as opposed to one owned by a common carrier or by a telephone company. Originally an organization's manual switchboard (operated by a person plugging cables into sockets) was known as a PMBX (Private Manual Branch eXchange). These were gradually replaced by automated electromechanical and then electronic switching systems, called PABXs (Private Automatic Branch eXchange). As PMBXes are almost unheard of, the terms PABX and PBX have become synonymous.