Rounded binary form is a slight variation on the simple binary that is often falsely identified as ternary form. The variation is found in the fact that the second section states a new musical idea "B" and then returns to the first idea "A." The diagram of this form would be A - (B A) as opposed to the typical A-B of a regular binary form.
Some people may think of this as ternary (3 part) form (A-B-A), but there are several differences between a true ternary and a rounded binary form. First, the "B" section of a true ternary form dramatically contrasts the musical statement of the "A" section. In rounded binary, the "B" section can have similar musical attributes to the "A" section. Second, the return of "A" in rounded binary is usually abbreviated (half of the original statement). In true ternary form the return of "A" is a complete restatement of the section.
This full restatement of the “A” section reinforces the fact that true ternary form consists of three complete sections. Each section contains its own separate and complete musical theme. In addition, each section contains its own separate and complete tonal center which ends in a full authentic cadence (final dominant to tonic resolution of harmonic motion). The separate identities of the “A” and “B” sections are usually highlighted by a distinct contrast in musical theme and tonal center. If the theme of the “A” section expresses the mood of a clam lullaby, the “B” section may be lively (either like a dance or march) or agitated. In addition, the new tonal center of the “B” section will relate to the original key in a contrasting way. The most common options are for the new key to be either a fifth above or the parallel minor/major.
The restatement of the “A” section does not always have to be an exact restatement. Although it is usually the same length, the original them can sometimes be decorated with additional ornamentation in the second statement. In this case, the final section is usually labeled as A’ in order to indicate the slight difference due to ornamentation.
This Learning Music With Ray video discusses ternary form. In it, I discuss the meaning and aspects of ternary form as they relate to musical form. I also point out the differences between ternary and rounded binary form. I then analyze a piece of music that is written in ternary form. In this stage of the lesson I review the components of this type of form, and the ways to identify them in the musical example.