An internal telephone exchange serving a single organization and having no connection with a public exchange. Also private automatic exchange (PAX). Other types of private exchange include:

1. Private automatic branch exchange (PABX): A private automatic telephone exchange that provides for the transmission of calls internally and to and from the public telephone network. Also private branch exchange (PBX).

2. Private automatic telex exchange (PATX): Provides telex service within and without an enterprise (largely obsolete).

3. Private manual branch exchange (PMBX): Private, manually operated telephone exchange that provides private telephone service to an organization and allows calls to be transmitted to or from the public telephone network.

4. Private digital exchange (PDX): Private exchange employing digital transmission techniques.