A phone connector is a family of cylindrically-shaped electrical connectors primarily for analog audio signals. Invented in the late 19th century for telephone switchboards, the phone connector remains in use for interfacing wired audio equipment, such as headphones, speakers, microphones, mixing consoles, and electronic musical instruments (e.g. electric guitars, keyboards, and effects units). A male connector (a plug), is mated into a female connector (a socket), though other terminology is used.
Plugs have 2 to 5 electrical contacts. The tip contact is indented with a groove. The sleeve contact is nearest the (conductive or insulated) handle. Contacts are insulated from each other by a band of non-conductive material. Between the tip and sleeve are 0 to 3 ring contacts. Since phone connectors have many uses, it is common to simply name the connector according to its number of rings: