Binary form
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Binary form is a way of structuring a piece of music.
Most strictly, a piece in binary form will be in two halves, equal in length. The first half will start in a certain key (or on a certain tonic), and end in a different key. The second half of the piece begins in the key that the first half ended in, and ends in the original key of the piece. The second half may also be repeated. If the key at the start was a major one, the key at the end of the first part will generally be the dominant of it (a fifth above), so that a piece beginning in C major will end the first half in G major. If the starting key is minor, the music will generally move to its relative major key, so if a piece starts in C minor, it will end the first half in E flat major. The first half is often repeated.
Binary form is sometimes characterised as having the form AB, although AA' is also frequently used. This second designation points to the fact that there is no great change in character between the two halves. The rhythms and melodic figures used will generally be closely related in each half, and if the piece is written for a musical ensemble, the instrumentation will generally be the same for both halves. This is contrast to the use of verse-chorus form in popular music. The contrast between the two halves is primarily one of the keys used.
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 is usually as the form of the theme in a set of variations. Many larger forms incorporate binary form, and many more complicated forms, such as sonata form, may share characteristics with binary form.
See: musical formja:二部形式