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.

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