Depending on the manufacturer of the thermostat or the heating/cooling equipment the 24 volt common wire could be the C, B or X terminal.

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Below are the standard thermostat connections.

Standard connections
R - 24 volts A/C power connection
RC – Two transformer system cooling 24 volt A/C connection
RH – Two transformer system heating 24 volt A/C connection
C – 24 volt common connection
G – Fan connection
Y or Y1 – First stage cooling connection
Y2 – Second stage cooling connection
W or W1 – First stage heating connection
W2 – Second stage heating connection
O – Reversing valve on for cooling connection
B – Revering Valve on for heating connection
E or Aux – supplemental heat connection
T - Outdoor anticipator reset
L - Service Light

Standard heat pump connections
Y – Compressor connection
O or B – Reversing valve connection