Three-part form, also called ternary form, is a sectional form consisting of three principal parts (A B A) in which each section is a complete musical statement.
The A sections of three-part forms are typically closed in formal design (they cadence on the tonic). Three-part forms are found in a wide variety of sizes. In the usual pattern, both A sections are at least a period in length. The B as well is usually at least a period in length, although it may sometimes be only a single phrase. short as three phrases to movements lasting many minutes. The function of the B part in this form is to provide contrast with the A part and to create a need to return to the A part to complete the pattern.
Expanded Ternary Form
The ternary form may be expanded by the use of repetitions of any section. The repeats may be made with double bars and repeat signs, or they may be written out if the composer wants to provide a different setting of the music when repeated. Such compositions are said to be in expanded ternary form.
The three-part form may be expanded by the use of auxiliary members, such as an introduction to precede the first A, a transition between the sections, or a coda after the final A part that serves to bring the composition to a close.
Rounded Binary Form
Many short compositions exhibit characteristics of both binary and ternary forms. These works are said to be in rounded binary or incipient three-part form. The rounded binary form differs from true binary form in that the first section (A) is repeated (sometimes in part) at the end of the second section. The rounded binary form differs from true ternary form in that the B section is not a complete section in itself, but is connected with the returning A material.