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Mozart's Sonata in D Major, K 284, Movement 3, Theme

    Notice that the theme begins in D major and proceeds to a half cadence at the first repeat sign.  Therefore, the harmonic motion may be characterized as proceeding from tonic to dominant and the section, offset from the succeeding section by a cadence, may be labeled A. The B section begins after the repeat sign and functions to close the theme in the tonic key, which it does with a suspended authentic cadence.

    This excerpt is an example of rounded binary form. Notice that the B section contains the theme from the A section as ending material. A characterization of the form would look like this: A - BA. This allows one to see that the return of A is incorporated into the B section, and not an actual return like in da capo form (ABA). Some rounded binary forms will have a more elaborate return of the A section. See the chapters on binary and ternary form for specific differences between the two.

    Some theorists may argue that the cadence at the first repeat sign is an authentic cadence in A major and that the harmony has modulated.  The arrival on A as tonic supports this; however, the function of the harmony is not a change of key, but rather a strengthening of D as tonic, a convention upheld by the form. The macroform of the entire theme, reduced to its basic harmonic progression, is I-V-I, where V tonicizes I. The decision regarding modulation depends on how modulation is defined, and theorists who would argue for modulation here may have a valid point.

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