Balanced audio
Two signal conductors with equal impedance to ground, so interference that hits both can be subtracted out at the receiving end.
A long cable is an antenna, and it picks up hum from mains wiring and fluorescent lights. Run two conductors tightly twisted together and they receive almost exactly the same noise. The input is a differential amplifier that only cares about the difference between them, so whatever is common to both — the common mode — drops out. How well it does that is measured as CMRR.
The cold leg often carries an inverted copy of the signal, but it's the impedance balance doing the work, not the mirror image. That is why a 100-metre run from stage to front-of-house can be silent while a six-metre unbalanced guitar lead already buzzes. XLR and TRS jacks are the usual connectors.