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The term musical form (or musical architecture) refers to the overall structure or plan of a piece of music; it describes the layout of a composition as divided into sections. 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."
According to Richard Middleton, musical form is "the shape or structure of the work." He describes it through difference: the distance moved from a repeat; the latter being the smallest difference. Difference is quantitative and qualitative: how far, and of what type, different. In many cases, form depends on statement and restatement, unity and variety, and contrast and connection.
The founding level of musical form can be divided into two parts:
The smallest level of construction concerns the way musical phrases are organized into musical sentences and "paragraphs" such as the verse of a song. This may be compared to, and is often decided by, the verse form or meter of the words or the steps of a dance.
For example, the twelve bar blues is a specific verse form, while common meter is found in many hymns and ballads and, again, the Elizabethan galliard, like many dances, requires a certain rhythm, pace and length of melody to fit its repeating pattern of steps. Simpler styles of music may be more or less wholly defined at this level of form, which therefore does not differ greatly from the loose sense first mentioned and which may carry with it rhythmic, harmonic, timbral, occasional and melodic conventions.
In the analysis of musical form, any components that can be defined on the time axis (such as sections and units) are conventionally designated by letters. Upper-case letters are used for the most fundamental, while lower-case letters are used for sub-divisions. If one such section returns in a varied or modified form, a numerical digit or an appropriate number of prime symbols appears after the letter. Even at this simplest level, there are patterns that may be re-used on larger timescales. For example, consider the analogy with rhyme schemes;
The following verse is composed of two differently-rhymed couplets (AABB), and thus its organization is binary or "twofold".
Contrast with the following verse, where the rhyme is repeated in the second line, followed by a variant in the third line - two half-lines sharing a new rhyme - and a return to the first arrangement in the last line (AABA), and thus its organization is song form. Ternary form or "threefold" is (ABA).
However, as music educator Stewart Macpherson stated, there is a preference at all levels of musical organization for groupings of two, four, eight over other divisions, so that even a ternary form is often extended by repetition of the first subject into a "fourfold" structure so that composers must guard against excessive "squareness".
The next level concerns the entire structure of any single self-contained musical piece. If the hymn, ballad, blues or dance alluded to above simply repeats the same musical material indefinitely then the piece is said to be in strophic form overall. If it repeats with distinct, sustained changes each time, for instance in setting, ornamentation or instrumentation, then the piece is a theme and variations. If two distinctly different themes are alternated indefinitely, as in a song alternating verse and chorus or in the alternating slow and fast sections of the Hungarian czardas, then this gives rise to a simple binary form. If the theme is played (perhaps twice), then a new theme is introduced, the piece then closing with a return to the first theme, we have a simple ternary form.
Great arguments and misunderstanding can be generated by such terms as 'ternary' and 'binary', as a complex piece may have elements of both at different organizational levels. A minuet, like any Baroque dance, generally had simple binary structure (AABB), however, this was frequently extended by the introduction of another minuet arranged for solo instruments (called the trio), after which the first was repeated again and the piece ended--this is a ternary form--ABA: the piece is binary on the lower compositional level but ternary on the higher. Organisational levels are not clearly and universally defined in western musicology, while words like "section" and "passage" are used at different levels by different scholars whose definitions, as Schlanker[full ] points out, cannot keep pace with the myriad innovations and variations devised by musicians.
The grandest level of organization may be referred to as "cyclical form". It concerns the arrangement of several self-contained pieces into a large-scale composition. For example, a set of songs with a related theme may be presented as a song-cycle, whereas a set of Baroque dances were presented as a suite. The opera and ballet may organize song and dance into even larger forms. This level of musical form, though it again applies and gives rise to different genres, takes more account of the methods of musical organisation used. For example: a symphony, a concerto and a sonata differ in scale and aim, yet generally resemble one another in the manner of their organization. The individual pieces which make up the larger form may be called movements.
Scholes suggested 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).
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. Sectional forms include:
Medley, potpourri or chain form is the extreme opposite, that of "unrelieved variation": it is simply an indefinite sequence of self-contained sections (ABCD...), sometimes with repeats (AABBCCDD...).
The term "Binary Form" is used to describe a musical piece with two sections that are about equal in length. Binary Form can be written as AB or AABB.  Using the example of Greensleeves provided, the first system is almost identical to the second system. We call the first system A and the second system A' (A prime) because of the slight difference in the last measure and a half. The next two systems (3rd and 4th) are almost identical as well, but a new musical idea entirely then the first two systems. We call the third system B and the fourth system B' (B prime) because of the slight difference in the last measure and a half. As a whole, this piece of music is in Binary Form: AA'BB'. 
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 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".
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.
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 (AA1BA2CA3BA4) or (ABA1CA2B1A).
This form, also known as sonata form, first movement form, compound binary, ternary and a variety of other names,[example needed] 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]A1ab1+coda).
Middleton, following Andrew Chester (1970) and Charles Keil (1987) suggests that forms in the context of popular music may be characterized on the one hand as "syntactic, embodied, extensional", which are "produced by starting with small components--rhythmic or melodic motifs, perhaps--and then 'developing' these through techniques of modification and combination", or on the other as "processual, engendered, intensional" music, which "starts with a framework--a chord sequence, a melodic outline, a rhythmic pattern--and then extends itself by repeating the framework with perpetually varied inflections to the details filling it in."
In the 13th century the song cycle emerged, which is a set of related songs (as the suite is a set of related dances). The oratorio took shape in the second half of the 16th century as a narrative recounted--rather than acted--by the singers.[clarification needed]