C-wire (thermostat common wire)

The C-wire (short for 'common') is the wire that completes a thermostat's low-voltage control loop: the 'R' wire supplies 24V power and the 'C' wire returns it, so together they give the thermostat a continuous power source. Older, simple thermostats ran on batteries or 'stole' a trickle of power through the system, which was fine for a basic on/off stat — but modern Wi-Fi/smart thermostats have screens and radios that need steady power, so they need a proper C-wire. Without one (or with a flaky one) a smart thermostat behaves erratically: dropping offline, failing to switch the AC, or not holding temperature — which is why so much thermostat-not-controlling trouble traces back to the common wire rather than the thermostat itself. If your wall wiring has no spare C-wire, the fixes are running one, using an add-a-wire adapter, or picking a stat that doesn't require it — a job worth confirming with a licensed electrician on the controls side before you buy a smart thermostat.