What are the four sections of an AABA form?
The letters name the music, not the order of the chords: every A is the same tune over nearly the same changes (only the last one ends differently), and B is something else. Each section is eight measures, so one trip through the form, called a chorus, is 4 × 8 = 32 measures.
| Section | Measures | What happens |
|---|---|---|
| A | 1–8 | The main tune. It ends on a turnaround that leads back to the top. |
| A | 9–16 | The same tune again, with the same chords and the same turnaround. |
| B (the bridge) | 17–24 | The contrast: new chords, and often a move to another key area. It ends on a dominant that sets up the return. |
| A | 25–32 | The main tune a last time. This time the final two measures settle on the home chord. |
What does the bridge do?
After two statements of the same tune, the bridge is the part that sounds like somewhere else. The chords leave the home key, often by a string of dominant chords or a short trip to the subdominant. In a string of dominants each chord is a fifth below the one before and is often held for two measures. Its job is to make the last A feel like a homecoming. It is also called the middle eight, the release or the B section.
What is the turnaround at the end of an A?
The last two measures of the first two A sections are a turnaround: a short chord pattern, often a ii–V or a ii–V–I, that points back at the home chord so the tune can start again. The last A has no need to go round, so it ends on the home chord instead. The ii–V–I entry explains the pattern itself.
How do you keep your place in a 32-bar form?
Count in eights, not in ones. Each A is the same music, so when the tune comes round again you are at measure 9, and when it comes round a third time you are at measure 25. Between them, the moment the chords leave home is the bridge arriving; you are then at measure 17, and eight measures from the last A. In a solo, one chorus is one trip through all 32 measures, and the next chorus starts straight away on the first A.
How is it different from the 12-bar blues and from verse and chorus?
The 12-bar blues is one short pattern of chords played round and round; AABA is a longer tune with one contrast in it. A verse–chorus pop song has two different sections that each return, and the chorus is usually its high point, while in AABA the return of the main tune is the point, and the bridge only passes through. Many songs are in neither form.