Honeywell RTH5100B Guide D'installation page 8

Installation Guide
Did you purchase right thermostat? Count wires to check.
Count the number of wires coming out of the wall and attached to
terminals in your old thermostat. If any of them are attached to a
terminal marked "C" or "C1" do not count that wire in your total.
If the total (not counting C or C1) is 5 wires or less, proceed to the
next page. If you have 6 or more wires, your new thermostat may
not be compatible with your system. Stop installation and call
1-800-468-1502 for advice.
Do you have 6 or more wires? If so, stop
now. You may have purchased the wrong
replacement thermostat.