The common wire sometimes referred to as the c wire is what s responsible for delivering constant power to your thermostat.
Ac thermostat control wiring.
The yellow wires terminate at your compressor contactor via an air handler.
The yellow wires connect to the y terminals on your thermostat.
If your home s system of this vintage has provisions for air conditioning cooling then the thermostat wiring will have at least three wires some will require a separate r terminal for heating and cooling and will be labeled rh and rc now needing a minimum of 4 wires r w and a y terminal.
G terminal is connected to the green wire or g wire this is for the blower fan in the air handler.
It terminates at the air handler or furnace.
Furthermore it will be one of the few control wires thermostat wires that will terminate at the condenser in a split system air conditioner and heating system.
Origin is the control transformer and then the r terminal.
They control your air conditioning system.
The green wire underneath your thermostat connects to the fan of your furnace or air handler.