Form is the overall, shape, organization, or structure of a musical composition. It refers to the succession of musical materials in a piece of music, including repetition and contrast, from its start to its finish. When diagramming musical form, the initial material is labeled as "A," and successive contrasting material is labeled with successive letters of the alphabet. In classical practice, lowercase letters are used to label individual phrases, and capital letters are used to label larger sections. In analyzing popular music, it usually suffices to use only capital letters (e.g., in describing the classic 32-bar song form as "AABA").