Here are the pictures, as requested: The light at the bottom of the stairs looks like There are three cables, all two conductors and the light itself. I will attempt to describe the connections. I'll call the top left cable as cable #1, the cable to its right cable #2, the bottom cable as cable #3. -white(1), white(3), and white of light wire nutted. -black(1), ***white(2)***, and black(3) wire nutted. -black(2) and black of light wire nutted. Switch #1 looks like: (I took lots of pictures, but none of them were really "easy to understand", like the fixture picture.) It looks like the bronze screw is in the middle and the adjacent screws are brass. Switch #2 looks like: It looks like the bronze screw is in the middle and the adjacent screws are brass. As far as I know, the job was professionally done. It works correctly, inasmuch as both switches can turn on the light. Given the age of the fixture, the style of the fixture, and the layers of paint, I would guess that this was the original fixture when the house was built. Does this help? I didn't do the voltmeter measurements, as I was confused by the instructions. Here are my guesses (based on other people's comments): 1) At the light fixture, the supply cable is the bottom cable. Cable #3, in my description. 2) At the light fixture, the cable going to switch #1 is the top right cable, Cable #2. 3) At the light fixture, the cable going to feed other things is the top left cable, Cable #1. 4) Normel pretty much summed it up in: "2 conductor cable is switch loop from the fixture; 3 conductor cable is between the two switches. This was my explanation way back in post #2." and "The white is not a neutral. You have the incoming hot and neutral at the light. The hot at the light is fed to the switch via the white wire, so the two wire cable is a switch loop. The screws where the two black wires attach are your commons." Texasredhead: The black wires are attached to each of the bronze screws. So, am I close in my guesses? Unfortunately, I have a followup question. I was looking at my book "Electrical code simplified house wiring guide, based on the 20th edition of the CEC and the BC admendments, 2006 to 2010" and saw this picture: In this picture, the middle switch shows the white wire going to the common terminal. Newbie/dummy question: Why is this diagram different from the way my setup is done? Am I missing something fundamental in my (mis)understanding?

No, you'd need a three-way switch with an occupancy sensor. They're made, of course, but generally a lot more expensive than $10. Changing the wiring is a little more complicated than disconnecting the red wire. Maybe you know that and I'm misunderstanding your message, otherwise you might want to read up on how a three-way circuit is wired. Google will give you a nice diagram that will explain it better than I could with words.

In addition to that when following the steps in the other post about trouble shooting a three way switch, i found out that with one switch out of the box completely ( no wires connected) and the power on the other three way switch could still control the light on and off. The wiring worked before and it was a three way not a four way switch that was installed, and I changed nothing but the two switches. I also ran a continuity test and swap both three way switches to see if that was the problem - it was not. I'm at a loss here as to how to proceed next. Thanks again.

You need to run the 12/3 between the switches and 12/2 from either swtich to and between the lights.