page_title: Phone connector (audio)
section_title: Less common
hierarchical_section_title: Phone connector (audio) / Historical development / Less common
caption_attribution_description: English: Double jack plug. An obsolete six-conductor plug consisting of two 1/4" TSR jack plugs at 1" spacing, from the personal collection of Andrew Alder. Such plugs were used in telephone exchanges, and this may from the tip profiles have been the intent of this example, but it came from a professional TV studio where it was used to access the patch points in a mono 15 IPS 1/4" tape recorder. Only the tips and bodies (sleeves) of the plugs were used in this application, as the patch points had no ring contacts.
context_page_description: A phone connector, also known as phone jack, audio jack, headphone jack or jack plug, is a family of electrical connectors typically used for analog audio signals. The standard is that a plug will connect with a jack.
  The phone connector was invented for use in telephone switchboards in the 19th century and is still widely used.
  The phone connector is cylindrical in shape, with a grooved tip to retain it. In its original audio configuration, it typically has two, three, four and, occasionally, five contacts. Three-contact versions are known as TRS connectors, where T stands for "tip", R stands for "ring" and S stands for "sleeve". Ring contacts are typically the same diameter as the sleeve, the long shank. Similarly, two-, four- and five- contact versions are called TS, TRRS and TRRRS connectors respectively. The outside diameter of the "sleeve" conductor is 6.35 millimetres. The "mini" connector has a diameter of 3.5 mm and the "sub-mini" connector has a diameter of 2.5 mm.
context_section_description: A two-pin version, known to the telecom industry as a "310 connector", consists of two phone ​¹⁄₄-inch phone plugs at a centre spacing of ⁵⁄₈ inch (16 mm). The socket versions of these can be used with normal phone plugs provided the plug bodies are not too large, but the plug version will only mate with two sockets at ​⁵⁄₈ inches centre spacing, or with line sockets, again with sufficiently small bodies. These connectors are still used today in telephone company central offices on "DSX" patch panels for DS1 circuits. A similar type of 3.5 mm connector is often used in the armrests of older aircraft, as part of the on-board in-flight entertainment system. Plugging a stereo plug into one of the two mono jacks typically results in the audio coming into only one ear. Adapters are available.
  A short-barrelled version of the phone plug was used for 20th century high-impedance mono headphones, and in particular those used in World War II aircraft. These have become rare. It is physically possible to use a normal plug in a short socket, but a short plug will neither lock into a normal socket nor complete the tip circuit.
  Less commonly used sizes, both diameters and lengths, are also available from some manufacturers, and are used when it is desired to restrict the availability of matching connectors, such as 0.210-inch (5.3 mm) inside diameter jacks for fire safety communication jacks in public buildings.
caption: A dual 310 patch cable, two-pin phone plug
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