Honeywell RTH5100B Guide D'installation page 10

Installation Guide
Label wires and remove old wallplate
Use a screwdriver to disconnect wires one by one. As you discon-
nect each wire, wrap it with the label matching the letter on your old
thermostat. (Adhesive labels are supplied in your thermostat package.)
Remove the old wallplate only after all wires are labeled. Be careful
not to let loose wires fall into the wall opening.
Wrap supplied labels securely around each wire.
Do not let wires fall into wall opening!
IGNORE WIRE COLORS: Use only
letter designations to identify wire types.