6.3 mm

6.35mm (or 6.3mm) connectors are often called phone connectors, phone plugs or phone jacks, and the connector originated in telephone exchange patch bays, from which the name comes. Today, phone connectors are mainly used in various types of musical equipment, e.g. in electric guitars, synthesizers, mixing consoles, amplifiers, etc. Another common area of ​​use is as a headphone connector, mainly in professional or Hi-Fi equipment. Low-end vocal microphones also commonly have a hard-wired telephone plug.

The outer diameter of the connector is 6.35mm, a 1/4”, and they are often called quarter inch jacks / plugs.

Phone connectors are available in two versions, mono, or stereo. The mono version (2-pole) is often called TS (Tip, Sleeve), while the three-pole (stereo) is called TRS (Tip, Ring, Sleeve). 1/4” TRRS connectors have never been used. 
As with other miniature phone jacks, the sleeve is always used for ground, and is normally connected to the plug housing (unless plastic or insulated). The tip is used for signal, and in balanced uses, tip is normally hot (+), with ring being cold (-). For headphones, tip is the left channel and ring has the right channel. Other than for headphones, 6.35mm connectors are normally not used for stereo signals, unlike 3.5mm connectors.