3 way switch

I have 2 3 way swithces off from a 75 year old house. All three wires are black, and I don't know which one is the common wire. Is there any way to find out? And does it matter which screws the other 2 wires are attached to if you have the correct common wire? Right now I have it hooked up and it works on both switches, sometimes..

If you disconnect all six wires, at one of the locations you will have one hot wire. It is the common on that side and the two travelers go to either of the other screws. After you connect the wires on that switch, check the three wires at the second location. One will now be hot. It is a traveler. Flip the installed switch to the other position. You will now have a different wire that is hot at the second location. It is your second traveler. The remaining wire is the common which goes to the light

Good explanation: I thought about that before reading your post and couldn't figure out how to describe it other than provide a link to a diyer site; which I also can't remember! Good post.

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