Gearing is the basis of clockmaking and the ability to calculate the necessary ratios is a fundamental clockmaking skill. To reduce pivot frictional losses, high gear ratios are necessary, leading in turn to losses in the gearing. Ratios above 10 to 1 require very high-numbered wheels to achieve a smooth gear action. The shape of gear teeth is carefully selected so that one tooth rolls on another - ideally, eliminating sliding friction. In clocks this is normally achieved by making the acting parts of wheel or pinion teeth of epicycloidal form (the curve generated by a point on the circumference of one circle rolling around another). In general engineering, the involute tooth shape is more generally used (the curve traced by a point on the diameter of a circle rolling on a plane). This tooth shape is used because of its strength and because it is possible to generate pinions or wheels of any number, using only one cutting hob. Involute pinions of less than 20 leaves cut with normal cutters have seriously weakened tooth roots, making involute gearing unsuitable for most clockmaking applications. Epicycloidal pinions have teeth with radial sides, giving stronger roots, and can be made with as few as five leaves. The best clocks have high-numbered pinions, with more than eleven leaves, but to obtain the necessary ratios they are more expensive to manufacture.