The term is associated with telephone usage. In twisted pair wiring to this day, the non-inverting and/or "live" wire of each pair is known as the tip, while the inverting and/or "earthy" wire is known as the ring. If the pair is shielded, or if the pair is accompanied by a dedicated earth wire, this third conductor is known as the sleeve. This usage corresponds to the connection to a three-connector jack plug in a manual telephone exchange. There are several competing theories as to how this usage originated.
The term tip ring sleeve is more common in some English-speaking countries than others. Outside of the USA the term stereo jack plug is probably more common, even for connectors not used for stereo. The modern profile three-conductor jack plug was originally designed for stereo signal connections, with left channel on the tip, right on the ring and common return on the body or sleeve. The term TRS is particularly appropriate to distinguish these three-conductor (stereo) plugs used in other than stereo applications.
For instance, when a TRS is used to make a balanced connection, the two active conductors are both used for a monaural signal. The ring, used for the right channel in stereo systems, is used instead for the inverting input. This is a common use in small audio mixing desks, where space is a premium and they offer a more compact alternative to XLR connectors. Another advantage offered by TRS connectors used for balanced microphone inputs is that a standard unbalanced signal lead using a mono jack plug can simply be plugged into such as input. The ring (right channel) contact then makes contact with the plug body, correctly grounding the inverting input.
TRS connectors are also commonly used as audio patch points, with the output on the tip (left channel) and the input on the ring (right channel). One advantage of this system is that the switch contact in the panel socket, originally designed for other purposes, can be used to make the circuit when the patch point is not in use. Another is that if the patch point is used as an output only, use of a mono jack plug correctly grounds the input. Use of 6.5mm TRS connectors in this way is seen in almost all professional and semi-professional audio mixing desks. In some, the concept is extended by using specially designed TRS jacks that will accept a mono jack plug partly inserted ("to the first click") and will then reliably connect the tip to the signal path without breaking it.
In some very compact equipment, 3.5mm TRS jacks are used as patch points.
Some sound recording devices use a TRS as a mono microphone input, using the tip as the signal path and the ring to connect a standby switch on the microphone.
Personal computer sound cards from Creative Labs, Sound Blaster or compatible to these use a 3.5mm TRS as a mono microphone input, and deliver a 5v polarising voltage on the ring to power electret microphones from the card manufacturer. Sometimes called phantom power, this is not a suitable power source for microphones designed for true phantom power and is better called bias voltage. Compatibility between different manufacturers is unreliable.
TRS connectors come in 6.5mm (1/4"), 3.5mm (miniature) and 2.5mm (subminiature) sizes. See also jack plug.