Headphone and earphone jacks on a wide range of equipment. 6.35 mm (1⁄4 in) plugs are common on home and professional component equipment, while 3.5 mm plugs are nearly universal for portable audio equipment. 2.5 mm plugs are not as common, but are used on communication equipment such as cordless phones, mobile phones, and two-way radios.
Consumer electronics devices such as digital cameras, camcorders, and portable DVD players use 3.5 mm connectors for composite video and audio output. Typically, a TRS connection is used for mono audio plus video, and a TRRS connection for stereo audio plus video. Cables designed for this use are often terminated with RCA connectors on the other end.
Hands-free sets and headsets often use 3.5 mm or 2.5 mm connectors. TRS connectors are used for mono audio out + an unbalanced microphone (with a shared ground). TRRS connectors are used to add an additional audio-out channel (i.e. stereo out + microphone).
Microphone inputs on tape and cassette recorders, sometimes with remote control switching on the ring, on early, monaural cassette recorders mostly a dual-pin version consisting of a 3.5 mm TS for the microphone and a 2.5 mm TS for remote control which switches the recorder's power supply.
Patching points (insert points) on a wide range of equipment.
Personal computers, sometimes using a sound card plugged into the computer. Stereo 3.5 mm jacks are used for:
Headphones/loudspeaker out (stereo)
Microphone input (mono, usually with 5 V power available on the ring. Note that traditional, incompatible, use of a stereo plug for a mono microphone is for balanced output)
Laptop computers generally have one line level jack for headphones and one mono jack for a microphone at microphone level. You can use an attenuating cable to convert line level or use a signal from an XLR connector, but it is not designed to record from a stereo device such as a radio or music player.
LCD monitors with built-in speakers will need a 3.5 mm male-male cable from the sound card.
Note: Higher end sound cards sometimes sport a breakout panel which supports 1⁄4 in plug devices as well.
Devices designed for surround output may use multiple jacks for paired channels (ex. TRS for front left and right; TRRS for front center, rear center, and subwoofer; and TRS for surround left and right). Circuitry on the sound device may be used to switch between traditional Line In/Line Out/Mic functions and surround output.
Electric guitars. Almost all electric guitars use a 1⁄4 in mono jack (socket) as their output connector. Some makes (such as Shergold) use a stereo jack instead for stereo output, or a second stereo jack, in addition to a mono jack (as with Rickenbacker).