Binary Form - Structure

Structure

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 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 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.