Binary form is perhaps the most simple of all the musical forms. The form is comprised of two sections and is sometimes seen with a recapitulation of the first section: ABA'.
The first section of a binary composition in a major key typically modulates to the dominant, thus displacing the centre of harmonic gravity to the fifth degree above the tonic: compositions in minor keys similarly modulate to the relative major (i.e., the major key centred on the third degree above the tonic). The second section begins in the new key and, after thriving for a period on the harmonic thus generated, returns to the home key. Binary structures, though not necessarily monothematic, tend to rely on closely linked melodic-rhythmic materials.
Rounded binary form features a complete recapitulation of the first section and is given the mark of ABA'. The start of the B section, in this type of Binary, is sometimes referred to as the bridge and concludes with a half cadence in the original key. Without the recapitulation of the opening section, the Binary is referred to as a simple Binary.
Sectional Binary features an Authentic Cadence in the tonic key. These type of binary forms generally have a different key for the sections. If the first section ends with any other cadence, the binary is referred to as continuous - meaning the B section continues with the new key established by the cadence.
Examples of Binary Form
- Corelli, Suite for Strings, "Badinerie"
- Beethoven, German Dances
Encyclopedia Britannica 
|Polyphonic forms||Canon • Canzona • Invention • Fugue • Organum • Ricercar • Round • Sinfornia|
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