3 way switches and how they work

I have the concept of how a three way switch works but am a little confused on one area. I had a home today that had a three way on the stairs. The only way to get the light to work was to have the bottom switch on and then the top one would work. I have already sent the report to the client but I was wondering if someone could explain why it is happening. I did put in the report that it was defective and an electrician would need to come out to rectify.

Someone mis-wired the switches and has a wire transposed on the common terminal

Either wired incorrectly or a regular single pole switch was used where a three way should be.


Correct and we run into this from time to time.

A common is swapped with a traveler

Thanks guys. I now have greater knowledge.

The main difference between a 3-way switch and single pole switch is the traveler.

There are several ways to wire a 3-way, it depends on where power originates and location of switches/light.


Great link Christopher, now i’m really learning!

Easy way to check with the original type of switches but not so with the newer flat or designer switches.

If it is the older / regular type of switch, a single pole switch will have on / off written on the switch. A 3 way or 4 way switch will be blank. It is the first thing I look for when I run into a hall or stair light that operates as described in the original post.