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. 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.
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.
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 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 section 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.