The basic form of songs (melody and harmony), usually 16 or 32 bars long, and usually made of a limited number of four- or eight-bar phrases that either repeat or differ. Letter names are used to describe how these phrases are grouped into melodic and harmonic sections, and help musicians see the pattern of repeating and differing phrases (AABA, ABAB, ABAC, etc.). In AABA form, the B section is sometimes called the "bridge," "middle eight," "release," "channel," or other names, and is a change from the A sections both melodically and harmonically. Many jazz pieces are in AABA. (Letter names can also be used to describe the four to eight-measure patterns within each section.)