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. It is, "the ways in which a composition is shaped to create a meaningful musical experience for the listener." Form refers to the largest shape of the composition. Form in music is the result of the interaction of the four structural elements described above [sound, harmony, melody, rhythm]." 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. In tonal harmony, form is articulated primarily through cadences, phrases, and periods. "Form refers to the larger shape of the composition. Form in music is the result of the interaction of the four structural elements," of sound, harmony, melody, and rhythm. Compositions that do not follow a fixed structure and rely more on improvisation are considered free-form. A fantasia is an example of this. Composer Debussy in 1907 wrote that, "I am more and more convinced that music is not, in essence, a thing that can be cast into a traditional and fixed form. It is made up of colors and rhythms." To aid in the process of describing form, musicians have developed a simple system of labeling musical units with letters. In his textbook Listening to Music, professor Craig Wright writes: The first statement of a musical idea is designated A. Subsequent contrasting sections are labeled B, C, D, and so on. If the first or any other musical unit returns in varied form, then that variation is indicated by a superscript number—A1 and B2, for example.