Binary/Ternary/Rondo/Theme and Variations

Binary Form

Binary form is a musical form in two related sections, both of which are usually repeated. In music this is usually performed as A-A-B-B. Referring to an A section (melody) repeated and a B section (melody) repeated.

Most strictly, a piece in binary form is characterized by two complementary, related sections of roughly equal duration, which come up frequently. The first section will start in a certain key, and will usually modulate (change key) to a related key:

compositions in major keys will typically modulate to the dominant, the fifth scale degree above the tonic
compositions in minor keys will typically modulate to the relative major, the major key centered on the third scale degree above the tonic; alternatively the first section could close in the dominant minor, or with an imperfect cadence in the original key.

The second section of the piece begins in the newly established key, where it remains for an indefinite period of time. After some harmonic activity, the piece will eventually modulate back to its original key before ending. More often than not, especially in 18th-century compositions, the A and B sections are separated by double bars with repeat signs, meaning both sections were to be repeated.

Binary form is usually characterized as having the form AB, though since both sections repeat, a more accurate description would be AABB. Others, however, prefer to use the label AA′. This second designation points to the fact that there is no great change in character between the two sections. The rhythms and melodic material used will generally be closely related in each section, and if the piece is written for a musical ensemble, the instrumentation will generally be the same.

Ternary Form

Ternary form is a three-part musical form where the first section (A) is repeated after the second section (B) ends. It is usually diagrammed as A-B-A.

In ternary form each section is self-contained both thematically as well as tonally (that is, each section contains distinct and complete themes), and ends with an authentic cadence. The B section is generally in a contrasting but closely related key, usually a perfect fifth above or the parallel minor of the home key of the A section (V or i).

Commonly, the third section will feature more ornamentation than the first section. In these cases the last section is sometimes labeled A' or A1 to indicate that it is slightly different from the first A section.

Rondo Form

Rondo is a term that has been used in music in a number of ways, most often in reference to a musical form, but also to a character type that is distinct from the form.

The term and formal principle may have derived from the medieval poetic form, rondeau, which contains repetitions of a couplet separated by longer sections of poetry.

In rondo form, a principal theme (sometimes called the "refrain") alternates with one or more contrasting themes, generally called "episodes," but also occasionally referred to as "digressions" or "couplets." Possible patterns in the Classical period include: ABA, ABACA, or ABACABA. These are sometimes designated "first rondo", "second rondo", and "third rondo", respectively. The number of themes can vary from piece to piece, and the recurring element is sometimes embellished and/or shortened in order to provide for variation.

Typical tonal structure of classical seven-part rondo, late 18th and early 19th centuries.

	A	B	A	C	A	B'	A
Major key	I	V	I	VI, IV or parallel minor	I	I	I
Minor key	I	III or V	I	VI or IV	I	I	I

Theme and Variations Form

In music, variation is a formal technique where material is repeated in an altered form. The changes may involve harmony, melody, rhythm, timbre, orchestration or any combination of these.

Theme and Variation forms are based specifically on melodic variation, in which the fundamental musical idea, or theme, is repeated in altered form or accompanied in a different manner. Theme and Variation structure generally begins with a theme (which is itself sometimes preceded by an introduction), typically between eight and thirty-two bars in length; each variation, particularly in music of the eighteenth century and earlier, will be of the same length and structure as the theme. This form may in part have derived from the practical inventiveness of musicians; "Court dances were long; the tunes which accompanied them were short. Their repetition became intolerably wearisome, and inevitably led the player to indulge in extempore variation and ornament"; however, the format of the dance required these variations to maintain the same duration and shape of the tune.

Variation forms can be written as 'free-standing' pieces for solo instruments or ensembles, or can constitute a movement of a larger piece. Most jazz music is structured on a basic pattern of theme and variations.

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