In the fifteenth century, spring-operated clocks appeared in Europe. Their mechanisms mainly used gears, but cams played a small but important role in the history of horology. The first application was in spring clocks called stackfreed, where a cam was used to maintain a constant driving torque exerted by the spring on the shaft of the clock hands. More sophisticated is the application of equation clocks, where a cam shaped to reproduce the equation of time is used according to the data provided by Huygens. The aim is to take into account the difference between the conventional twenty-four-hour day and the true solar day. The reciprocating motion that a cam imparts to the follower is similar to that of a player on certain musical instruments. This suggested replacing the human performer with small cams arranged on a cylinder. This idea is the basis of mechanical music. The cams used in mechanical instruments are also known as spikes, nails, needles, or pegs. The oldest instrument to use this system is the simple carillon. In pipe organs, cams are more complex because they must allow for notes of different durations. The orchestrion, fair organ, and barrel organ were effective in spreading music throughout the world. Musical boxes were another popular form of mechanical music.