In music theory, the division of music into parts occurs when you link two or more periods that sound like they belong together. They share major harmonic focal points, similar melody lines, and similar rhythm structure. They may have other resemblances, too. Parts can be further linked together to create musical forms.
Composers conventionally give alphabetic labels to the musical parts within a composition: A, B, C, and so forth. If a part is repeated in a song, its letter also is repeated. For example, ABA is a familiar layout in classical music, where the opening theme, or the main musical idea that runs through a song (labeled A), after vanishing during part B, is repeated at the end of the song.
As the contrast form, where you have different musical sections that can differ widely from one another, AB forms come in a boundless array of possibilities. You may see recurring sections, unique ones, or any combination of both. For example, a rondo — a popular form in classical music — alternates between a recurring section and others that occur one time each. A rondo, then, would be labeled ABACADA. . . (and so on).
You may even encounter an ongoing form, which has no recurrence whatsoever: ABCDE. . . .This form creates what’s known as a throughcomposed piece of music.
One‐part form (A)
The one‐part form, also known as A form or unbroken form, is the most primitive song structure and is also sometimes referred to as the air form or ballad form. In a one‐part form, a simple melody is repeated with slight changes to accommodate different words, as in a strophic song like “Old McDonald Had a Farm.” This song repeats the same musical line but changes the words with each verse.
The one‐part form is mostly found in folk songs, carols, or other songs that are short and have a limited theme and movement. A forms come only in a single variety. They may be long or short, but they’re always described as A (or AA, or even AAA).
Binary form (AB)
Binary form consists of two contrasting sections that function as statement counterstatement. The pattern may be a simple AB, as in “My Country, ’Tis of Thee,” or in simple minuets, where the form is usually AABB, with the second A and second B being variations of the first A and B.
In the binary form used in the Baroque period, the pattern can involve a change of key, usually to the key of the fifth of the original key if the piece is in a major key. Part A begins in one key and ends in the key of the fifth, while part B begins in the new key and ends in the original key. Each part is repeated, giving the pattern AABB.
Three‐part form (ABA)
Songs frequently take the form ABA, known as three‐part form or ternary/tertiary form. This simple form is produced by varying and repeating the melody. For example, “Twinkle, Twinkle, Little Star” states a tune, varies it, and then restates it (which makes it ABA form). The B part here may be called the bridge, or the link, between the two A parts.
Here’s how the three‐part song form works:
The first part, A, may be played once or repeated immediately.
The middle part, B, is a contrasting section, meaning it’s different than the first section.
The last part is the same or very similar to the first part, A.
Three‐part ABA form extends the idea of statement and departure by bringing back the first section. Both contrast and repetition are used in this form. Pop music is frequently a variation on ABA, called AABA, while blues is often AAB. AABA form is used in songs like “Over the Rainbow.”
Arch form (ABCBA)
Music written in arch form is made up of three parts: A, B, and C. In arch form, the A, B, and C are played sequentially, and then part B is played a second time, directly following the C, and the song ends with the replaying of the A part.
Composer Béle Bartók used this form in many of his pieces, including his Piano Concerto No. 2 and Violin Concerto No. 2. More recent examples include Steve Reich’s “The Desert Music” and Hella’s “Biblical Violence.”