To understand how a three-way switch works, you need to pay attention to which wire goes to which screw. The green or grounding screw is attached to the bare copper or green wires. They are the grounding wires. The grounding screws are a new addition to the three-way switches. That is why you may not find them on older 3-way switches.
Then there are the traveler screws that are connected to the hot or charged wire. These screws will provide two different paths for the electricity to travel to the fixture.
Lastly, a common terminal is darker in color than the traveler terminals. It either connects to the hot wire that leads to the fixture or accepts the hot wire coming from the power source.
If the circuit’s power is on, any of these may be hot depending on how the switches are toggled. The 3-way switch is designed to interrupt the hot wire when it’s turned off. The white wire coming straight from the power source goes uninterrupted into the light fixture while the hot wires are routed through the switches.
