Swing arrangements mainly consisted of composed sections, but with certain sections designated for improvisation. A bebop tune, however,, would simply consist of a statement of the head, or main theme, extended solos over the head’s harmonic structure, and then one final statement of the head. It was common for bebop musicians to compose new, complex melodies over well-known chord progressions. One example of this is Charlie Parker’s “Ornithology,” which is based on the changes from “How High the Moon,” a popular show tune in the 1940s.