A small round connector for accepting the pin-shaped plug from a standard phone headset. This jack can be also be used with accessories such as a wired hands-free car kit with speakerphone.
While 2.5mm is the standard for phone headsets, the slightly larger 3.5mm size is the standard for music headphones.
Either size can support stereo sound and/or a microphone, depending on the number of separate connenctor rings on the plug and jack.
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