In music, form refers to the structure of a musical composition or performance. In his book, Worlds of Music, Jeff Todd Titon suggests that a number of organizational elements may determine the formal structure of a piece of music, such as "the arrangement of musical units of rhythm, melody, and/or harmony that show repetition or variation, the arrangement of the instruments (as in the order of solos in a jazz or bluegrass performance), or the way a symphonic piece is orchestrated", among other factors.
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These organizational elements may be broken into smaller units called phrases, which express a musical idea but lack sufficient weight to stand alone. Musical form unfolds over time through the expansion and development of these ideas.
Compositions that do not follow a fixed structure and rely more on improvisation are considered free-form. A fantasia is an example of this.