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Music rotation is a practice under the jurisdiction of the music director at a radio station. Generally occurring either a once a week or on all weekdays, the oldest music (generally sorted by genre) is replaced with newer music a DJ has picked out of the new bin to review. Station DJs will review an album and write in a level of desired rotation (generally Light, Medium, Heavy) at which point the music director will then decide whether or not that CD is put into rotation or simply added to the station's library. Labels hire firms to call college radio stations and make sure their CDs are put into rotation, or at least reviewed, since an album put directly into the library will rarely be played, which leads to questions raised by commercialism for many stations.
See also Spin (radio).