There are actually two major kinds of binary form: that used during the Baroque Period and the more advanced form used in the Classical Period. The discussion here will use a few harmonic terms which are covered in Lesson 8 of our music theory section.
The Baroque binary form, also called simple binary, is
divided into two sections, which we'll call "a" and "b". The "a"
section begins in I (the tonic) and usually moves to V (dominant)
in major or III (mediant) in minor. This sets up the "b" section,
which begins where the "a" section left off and brings the music
back to the tonic. The "a" and "b" sections usually contain
different musical material and themes. Both sections are also
repeated, so the piece is something like:
The Classical binary form, called rounded binary, simply
adds a little bit to the simple binary form. Rounding is
when, at the end of the second section, a strong thematic reference
is made to the beginning of the movement. Otherwise, the harmonic
shape is just like that of the simple binary. This would look
Binary form was popular throughout the Baroque and into the
early Classical Periods. It then developed into the sonata form, which basically dominated
musical form for almost the next two centuries.