It is very important to first understand exactly how a 3-way system operates. If you follow the simple line drawing above you can see that by following the solid line from the hot wire all the way to the light it makes a complete circuit. The solid line in the switch represents the switch being in one position while the dotted line shows the alternate position the switch can be turned to. You can see by flipping either switch the circuit would open and the light would turn off.
This is a simple schematic and does not show how the actual wires would run. I will show some of those in the following diagrams. In some cases there may be more than 2 locations to turn on a light or lights. When 3 or more switches turn on the same light or lights a 4-way switch is required.
In the wiring diagram above we can see the 4-way switch installed between the two 3-way switches. You can put multiple 4-way switches in between two 3 ways. Notice in this diagram you can see the actual operation of the switches. The solid line is the actual switch position while the dotted lines show the alternate position.
With a 3-way switch system there is a number of variables that make troubleshooting very difficult for a novice. The most common mistake is made when a switch is simply replaced. The common screw on a 3 way switch has to be connected to the proper wire or the system will not work correctly. The basic premise is always the same. In the diagram below you can see one common is the hot wire and the other common goes to the light. It doesn’t matter on which switch these go but they must go to the common screw. On most 3-way switches this is the black screw with the other two screws being brass colored. The two traveler wires go only from one switch to the other. It doesn’t matter which of the two screws they go to on each switch as long as it is one of the 2 brass colored screws.
here
