What is a Jack plug?

The jack plug or phone (not phono) plug is a commonly used audio connector.   It has a long cylindrical shaft with two or more contacts.  It was invented for use in telephone switchboards in the 1800s and is still widely used, both in its original 6.3mm (1/4 inch) size and in smaller versions.

Modern jack plugs are available in three standard sizes, 6.35mm (1/4"), 3.5mm (1/8") and 2.5mm (3/32").

The original 6.35 mm (1/4") jack plug dates from 1878, for use in human operated telephone exchanges - making it one of the oldest electrical signal connector standards still in use.

The 3.5 mm or miniature and 2.5 mm or subminiature sizes were originally designed as two-conductor connectors for earpieces on transistor radios. All three sizes are now readily available in two-conductor (mono) and three-conductor (stereo or tip ring sleeve) adaptations.

Four and five conductor versions of the 3.5mm jack plug have been developed for particular applications. A four conductor version is becoming the standard output connector for compact camcorders, providing stereo audio plus a video signal. This interface is also seen on some laptop computers.

Proprietary versions of the 3.5mm jack plug using four or five conductor versions exist, such as the audio connector on the first four generations of Apple's iPod portable MP3 player (the 5th generation iPod uses a standard 3 conductor cable), where the extra conductors are used to supply power for accessories.

There is also an optical connector used for TOSLINK digital audio signals that is the same size as a 3.5mm jack plug. Dual function sockets have been developed that can make either an optical connection to such a plug or an electrical connection to a stereo jack plug.

What is a Jack plug used for?

The jack plug is widely used for connecting a large variety of audio equipment.  Some common applications are listed below:

Headphone and earphone jacks - 6.35mm jack plugs are often used on recording studio and home hi-fi equipment, while 3.5mm plugs are nearly universal for consumer portable audio equipment. 2.5mm jack plugs are less common, but can be found on communications equipment such as two-way radios and mobile phones.

Microphones - used as inputs on audio recording equipment, sometimes with remote control switching on the ring connection.

Patching - in professional recording studios the 6.35mm jack plug is used in patching points on a wide range of equipment, including mixing desks, samplers, and patching racks.

Personal computer sound cards - for audio input, output as well as headphone outputs and microphone inputs.  On consumer level sound cards the 3.5mm jack is normally used.

Electric guitars - almost all electric guitars use a 6.35mm mono jack (socket) as their output connector.  Instrument amplifiers for guitars, basses and similar amplified musical instruments universally use 6.35mm jacks.  Commonly called a guitar cord or a patching cord, the first name reflecting this usage, the second the history of the jack plug's development for use in manual telephone exchanges.

Loudspeakers -  jack plugs can be found used for connecting speakers with amplifiers, especially on low to medium end equipment. Speakon connectors are generally considered to be superior for this application and so are usually preferred on higher-end equipment, although it is not uncommon to find both connectors provided for compatibility.

Foot switches and effects pedals - Stereo jack plugs are used for double switches (for example by Fender). There is little compatibility between makers.

Professional music equipment - the majority of professional audio equipment uses mono jacks as the standard unbalanced input or output connector, often providing a 6.35mm unbalanced line connector alongside as an alternative to an XLR balanced line connector.

6.35mm jack plugs are also used on effects equipment, electronic keyboards, effects pedals, sustain pedals, mixing desks and many other devices.  For example, electronic drum kits use 6.35mm jacks to connect the sensor pads to a synthesizer module. In this usage, a change in voltage on the wire indicates a drum hit.

Photographic studio strobe lights - often use 6.35mm or 3.5mm jacks for the flash synchronization input. A camera's electrical flash output (PC socket or hot shoe adapter) is cabled to the strobe light's sync input jacks. Some examples: Calumet Travelite, and Speedotron use a 6.35mm mono jack as the sync input; White Lightning uses 6.35mm stereo jacks; Pocket Wizard (radio trigger) and Alien Bees use 3.5mm mono jacks.