Binary form
Binary form is a musical form in 2 related sections, both of which are usually repeated. Binary is also a structure used to choreograph dance. In music this is usually performed as A-A-B-B. Binary form was popular during the Baroque period, often used to structure movements of keyboard sonatas. It was also used for short, one-movement works. Around the middle of the 18th century, the form largely fell from use as the principal design of entire movements 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, or the Minuet, Scherzo, or Trio sections of a "minuet and trio" or "scherzo and trio" movement in a sonata, symphony, etc. Many larger forms incorporate binary structures, and many more complicated forms (such as the 18th-century sonata form) share certain characteristics with binary form. Structure A typical example of a piece in binary form has two large sections of roughly equal duration. The first will begin in a certain key, which will often, (but not always), modulate to a closely related key. Pieces in a major key will usually modulate to the dominant, (the fifth scale degree above the tonic). Pieces in a minor key will generally modulate to the relative major key, (the key of the third scale degree above the minor tonic), or to the dominant minor. A piece in minor may also stay in the original key at the end of the first section, closing with an imperfect cadence. 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. 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. Further distinctions A piece in binary form can be further classified according to a number of characteristics: Simple vs. rounded 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