A riff is a brief, repeated melodic and/or rhythmic phrase that is sufficiently distinctive to influence the character and structure of a piece. It frequently begins the piece and then continues to become part of the accompaniment. Riffs typically range in length from one to four measures and can range in complexity from a single instrument to multiple instruments. A hook can resemble a riff in that it can be a short, catchy musical idea, but differs in that is is repeated much less frequently and thus attracts more attention when it does occur. A melody differs from a riff in that it is generally much longer, repeated less often, and, most importantly, is unique to that particular piece. Riffs are most commonly used in popular music but are also found in some classical music. They are similar to ostinatos, although the former term is commonly applied to popular music and the latter to classical music. A frequently cited example in classical music is Ravel's Boléro.