Form in music refers to the structure of a piece. It is the skeleton of music. The form of a piece of music defines how many sections it has, the relative size of each section, and how they relate to one another. Form determines when certain themes in a piece of music are heard and in what fashion that are heard. It is what gives music its shape.
Over the centuries, composers have experimented with form and structure. Those forms they found to be successful were reused, becoming standard and recognisable templates for writing new music.
Perhaps the most successful musical form is the Sonata form. Composers have found sonata form to be so useful that it became the basis of not only the Sonata, but also the Symphony, the String Quartet, and a variation of sonata form for the classical Concerto.
Other common musical forms include; Binary – a two section form where each section differs from the other (A-B), Ternary – three sections where the first two are the different and the first is repeated at the end (A-B-A), and Rondo – a multi section form with a repeating section interspersed with differing episodes (A-B-A-C-A, though many variants exist).
Follow the links below two learn more about these forms and others.