Serrated Washer - External
Shakeproof washers are an anti-vibration type of locking washer. The surface of the washer features special teeth that lock when tightened, making them resistant to vibration. They are sometimes referred to as lock tooth washers.
On an external shakeproof washer the teeth are found on the outside rim of the washer.
Shakeproof washers are most commonly used in the electrical industry and are often used on larger fasteners where it does not matter so much if the teeth are visible.