A standard for signal interconnections, widely used in synthesizers and particularly modular synthesizers. The phone jack standard was, as the name suggests, originally developed for use in telephone switchboards. The male jack is cylindrical with a pointed tip and a grooved band near the tip; it has an insulator that separates the tip area from the barrel of the cylinder (referred to as the "sleeve"). In use, interconnection cords or patch cords always have male connectors at both ends, and the equipment being connected always has female connectors.
There are actually three sizes of standard phone jacks, but only two are used in synths. The largest size has the male plug with a barrel diameter of 1/4" (6.35 mm) and is always referred to as the "quarter inch" jack. This size is commonly used for audio inputs and outputs on synthesizers and other musical equipment, and is the standard size for patch interconnection jacks in the Dotcom and MOTM modular synth formats. The next smallest size has the male plug with a barrel diameter of 3.5mm (it is sometimes referred to incorrectly as the "1/8 inch" jack). In synthesizers, this size is used mainly as the standard for patch interconnect in the Eurorack and Frac modular synth formats. (A third size with a male plug barrel diameter of 2.5mm is rarely seen in any musical gear.)
Phone plugs have the advantages of being inexpensive and fairly reliable for analog signal connections, and quick to connect and disconnect. The 1/4" size is known for being very sturdy and tolerating a lot of abuse; the main disadvantage is that the jacks take up a lot of space on a panel. The 3.5mm size takes up less space but tolerates less abuse. A feature of all types is that the female jacks can be arranged so that they have internal normalled connections, which is a "default" connection that is in effect when no plug is inserted in the jack.
Phone plugs and jacks may have two, three, or four separate contact surfaces. The two-contact type is known as "TS" (tip-sleeve) and is what is most commonly used with synths. The three-contact type is known as "TRS" (tip-ring-sleeve); it is commonly used in studios to support "balanced line" connections. Some synths have balanced-line outputs and have jacks that will provide a balanced line when a cord with a TRS plug is used, or a conventional unbalanced line when a cord with a TS plug is used. The four-contact "TRRS" type is seldom used in musical equipment.