Wiring Compatibility

[Afroz1]

While changing my old thermostat to New Nest Learning Thermostat, I observed that I have 4 wires. Green (connected to G). Red (connected to Rc and R), White (connected to W), Black (connected to Y). 

I am not understand the black wire in Y. Please help. 

[Patrick_Caezza]

@Afroz1 

There is no color standard for the thermostat wires.

Just match the terminals on your original thermostat to the Nest thermostat. Don't worry about the jumper between Rc and R