In music, form describes how the overall structure or plan of a song or piece of music, and it describes the layout of a composition as divided into sections. In the early 20th century, Tin Pan Alley songs and Broadway musical songs were often in AABA 32 bar form, in which the A sections repeated the same eight bar melody (with variation) and the B section provided a contrasting melody and/or harmony for 8 bars. From the 1960s onward, Western pop and rock songs are often in verse-chorus form, which is based around a sequence of verse and chorus ("refrain") sections, with new lyrics for most verses and repeating lyrics for the choruses. Popular music often makes use of strophic form, sometimes in conjunction with the twelve bar blues.

In the tenth edition of "The Oxford Companion to Music", Percy Scholes defines musical form as "a series of strategies designed to find a successful mean between the opposite extremes of unrelieved repetition and unrelieved alteration." Examples of common forms of Western music include the fugue, the invention, sonata-allegro, canon, strophic, theme and variations, and rondo.

Scholes states that European classical music had only six stand-alone forms: simple binary, simple ternary, compound binary, rondo, air with variations, and fugue (although musicologist Alfred Mann emphasized that the fugue is primarily a method of composition that has sometimes taken on certain structural conventions.)

Where a piece cannot readily be broken down into sectional units (though it might borrow some form from a poem, story or programme), it is said to be through-composed. Such is often the case with a fantasia, prelude, rhapsody, etude (or study), symphonic poem, Bagatelle, impromptu, etc. Professor Charles Keil classified forms and formal detail as "sectional, developmental, or variational."


This form is built from a sequence of clear-cut units that may be referred to by letters but also often have generic names such as introduction and coda, exposition, development and recapitulation, verse, chorus or refrain, and bridge. Introductions and codas, when they are no more than that, are frequently excluded from formal analysis. All such units may typically be eight measures long. Sectional forms include:


This form is defined by its "unrelieved repetition" (AAAA...).


Medley, potpourri is the extreme opposite, that of "unrelieved variation": it is simply an indefinite sequence of self-contained sections (ABCD...), sometimes with repeats (AABBCCDD...). Examples include orchestral overtures, which are sometimes no more than a string of the best tunes of the musical theatre show or opera to come.


This form uses two sections (AB...), each often repeated (AABB...). In 18th-century Western classical music, "simple binary" form was often used for dances and carried with it the convention that the two sections should be in different musical keys but same rhythm, duration and tone. The alternation of two tunes gives enough variety to permit a dance to be extended for as long as desired.
b


This form has three parts. In Western classical music a simple ternary form has a third section that is a recapitulation of the first (ABA). Often, the first section is repeated (AABA). This approach was popular in the 18th-century operatic aria, and was called "da capo" (i.e. "repeat from the top") form. Later, it gave rise to the 32-bar song, with the B section then often referred to as the "middle eight". A song has more need than a dance of a self-contained form with a beginning and an end of course.


This form has a recurring theme alternating with different (usually contrasting) sections called "episodes". It may be asymmetrical (ABACADAEA) or symmetrical (ABACABA). A recurring section, especially the main theme, is sometimes more thoroughly varied, or else one episode may be a "development" of it. A similar arrangement is the ritornello form of the Baroque concerto grosso. Arch form (ABCBA) resembles a symmetrical rondo without intermediate repetitions of the main theme. It is normally used in a round.


Variational forms are those in which variation is an important formative element.

Theme and Variations: a theme, which in itself can be of any shorter form (binary, ternary, etc.), forms the only "section" and is repeated indefinitely (as in strophic form) but is varied each time (A, B, A, F, Z, A), so as to make a sort of sectional chain form. An important variant of this, much used in 17th-century British music and in the Passacaglia and Chaconne, was that of the ground bass – a repeating bass theme or "basso ostinato" over and around which the rest of the structure unfolds, often, but not always, spinning polyphonic or contrapuntal threads, or improvising divisions and descants. This is said by Scholes (1977) to be the form "par excellence" of unaccompanied or accompanied solo instrumental music. The Rondo is often found with sections varied (AABACABA) or (ABACABA).


Developmental forms are built directly from smaller units, such as motifs. A well-known Classical piece with a motif is Beethoven's fifth symphony, which starts with three short repeated notes and then a long note. In Classical pieces that are based on motifs, the motif is usually combined, varied and worked out in different ways, perhaps having a symmetrical or arch-like underpinning and a progressive development from beginning to end. By far the most important developmental form in Western classical music is Sonata form.

This form, also known as "sonata" form, first movement form, compound binary, ternary and a variety of other names, developed from the binary-formed dance movement described above but is almost always cast in a greater ternary form having the nominal subdivisions of "Exposition, Development" and "Recapitulation". Usually, but not always, the "A" parts (Exposition and Recapitulation, respectively) may be subdivided into two or three themes or theme groups which are taken asunder and recombined to form the "B" part (the development) – thus e. g. (AabB[dev. of a and/or b]Aab+coda). This developmental form is generally confined to certain sections of the piece, as to the middle section of the first movement of a sonata, though 19th-century composers such as Berlioz, Liszt and Wagner made valiant efforts to derive large-scale works purely or mainly from the motif.

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