A toothed lock washer, also known as a serrated washer or star washer,^[8] has serrations that extend radially inward or outward to bite into the bearing surface. This type of washer is especially effective as a lock washer when used with a soft substrate, such as aluminium or plastic,^[8] and can resist rotation more than a plain washer on hard surfaces, as the tension between washer and the surface is applied over a much smaller area (the teeth). There are four types: internal, external, combination, and countersunk. The internal style has the serrations along the inner edge of the washer, which makes them more aesthetically pleasing.^[12] The external style has the serrations around the outer edge, which provides better holding power, because of the greater circumference.^[13] The combination style has serrations about both edges, for maximum holding power.^[14] The countersunk style is designed to be used with flat-head screws.