Hello,
I bought a new Honeywell thermostat and it explains how to change all the wires but my old thermostat only has two wires, black and white, but no corresponding letters. It's probably at least 20-30 yrs. Old. What do I do?

Red

Just hook the wires one to W and the other to R or RH depending upon how your new thermoststat has the letter listed.

Just hook the wires one to W and the other to R or RH depending upon how your new thermoststat has the letter listed.

The only available letters are "W, Y & G. The Rh and Rc have something in them, like a wire spanning both with red coating, called a "jumper" in the book

Do just as I said before. Loosen the screw and slip the wire under the RH terminal then put the other wire under the W terminal and you are done. Now if you want to have some fun read the directions in the papers supplied with the new thermostat. It will tell you to do the same thing for a heat only circuit.