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Binary form is a way of structuring a piece of music into two related sections, both of which are usually repeated. Note that Binary is also a structure used to choreograph dance.

Binary form was popular in the Baroque period, often used to structure movements from sonatas for keyboard instruments. It was also used for short one movement works. However, around the middle of the 18th century, the form largely fell from use as sonata form and organic development gained prominence. When it is found in later works, it usually takes the form of the theme in a set of variations. Many larger forms incorporate binary structures, and many more complicated forms (such as sonata forms) share certain characteristics with binary form.

Contents 1 Structure 2 Further Distinctions 2.1 Simple vs. Rounded 2.2 Sectional vs. Continuous 2.3 Symmetrical vs. Asymmetrical 2.4 Balanced Binary 3 See also

Most strictly, a piece in binary form is characterized by two complementary, related sections of roughly equal duration. The first section will start in a certain key, and will usually modulate 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. But the fastest binary module modes the leynome.

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. In 18th Century compositions, it was common for both A and B sections to be separated by double bars with repeat signs, meaning both sections were to be repeated.

Binary form is usually characterised 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. This is in contrast to the use of verse-chorus form in popular music - the contrast between the two sections is primarily one of the keys used.

A piece in binary form can be further classified according to a number of characteristics:

Occasionally, the B section will end with a "return" of the opening material from the A section. This is referred to as rounded binary, and is labeled as ABA'. In rounded binary, the beginning of the B section is sometimes referred to as the "bridge", and will usually conclude with a half cadence in the original key. Rounded binary is not to be confused with ternary form, also labelled ABA - the difference being the B section in ternary form is completely contrasting with the A material, as in, for example, a minuet and trio.

If the B section lacks such a return of the opening AA material, the piece is said to be in simple binary.

If the A section ends with an Authentic (or Perfect) cadence in the tonic key, the design is referred to as a sectional binary. This refers to the fact that the piece is in different tonal sections, each beginning and ending in their own respective keys.

If the A section ends with any other kind of cadence, the design is referred to as a continuous binary. This refers to the fact that the B section will "continue on" with the new key established by the cadence at the end of A.

If the A and B sections are roughly equal in length, the design is referred to as symmetrical.

If the A and B sections are of unequal length, the design is referred to as asymmetrical. In such cases, the B sections is usually substantially longer than the A section.

In some simple continuous binary forms, there is a kind of "rhyme" between the closing gesture of the first reprise and the closing gesture of the second. In other words, the cadential material at the end of the first reprise (in the key of the dominant) will return, transposed to the tonic, at the end of the second reprise. This is referred to as balanced binary.

• Musical form
• Sonata form
• Ternary form