what is binary form in music

Binary form is a musical form that consists of two related sections, usually labeled A and B, both of which are usually repeated. The form is characterized as having the structure AB, though since both sections repeat, a more accurate description would be AABB. The two sections are usually related in terms of rhythm and melodic material, and if the piece is written for a musical ensemble, the instrumentation will generally be the same. Binary form is usually performed as A-A-B-B, and the key concept to grasp is that there is some sort of contrast between the A section and the B section, which is usually created by a key change. There are two main types of binary form: simple binary form and rounded binary form. Simple binary form refers to the basic binary form structure, which has an A section followed by a B section in a different key, and both sections are usually repeated. Rounded binary form is very similar to simple binary form, except that section B is often longer than section A, and part of section A is repeated again at the end of section B.