A three way switch in my hallway went bad today.
There were three terminals on the old one, three on the new one, I carelessly assumed they were the same and transferred the wires.
Nothing was marked, but it turned out the old switch had the common on the upper left and the two travelers on the right upper and lower, while the new switch had the common on the lower right and the travelers on the upper right and left.
When I put the switch in wrong and it didn't work, I put a meter on it and found how it was wired. Obviously I should have done that first.
When did switches change configuration? Or have they always had that variation?