EXTERNAL

A hardened circular washer with twisted prongs or teeth which extend out from the outer edge of the washer.

This is the most preferable of all tooth-lock washers. It offers the greatest locking efficiency since the teeth are on the largest radius and provide greater torsional resistance. 

INTERNAL

A hardened circular washer with twisted prongs or teeth which extend inwards from the inside edge of the washer.

This is preferred when finished appearance is crucial and the teeth must be hidden under the head of the screw.