Modulation

In Western tonal music, a modulation is an established change of key or tonal center. The new scale or pitch collection must be present, and there is frequently a cadential figure in the tonic. A shift between keys with the same tonic (for example, C major and C minor) is considered a modal shift rather than a modulation, because the tonal center and much of the hierarchical system remains the same.

A modulation may be contrasted with a tonicization, in which there is a brief emphasis on a different tonal area, without resulting in a complete shift of tonal center. These two terms are difficult to define precisely, as it is not always possible to determine whether a new tonal center has been firmly established, or whether it is simply emphasized. One criterion may be the duration of focus on the new tonal center, and another may be how strongly the new tonal center is confirmed via harmonic progressions and cadences.

One frequent technique for diatonic modulation involves the use of a pivot chord, a chord that shares a common root and quality between the home key and the goal key to create a smooth transition between the tonal centers. If two tonal areas are closely related, they will share more potential pivot chords than if they are distantly related. In practice, the most likely pivot chords are those that serve a predominant function (the supertonic or subdominant) in the new key; theorists advise against (and composers avoid) using a chord with a strong tonic or dominant function in one or both keys as a pivot chord.

Another technique for modulation is a chromatic modulation, which relies on introducing a chromatic tone present in the new key but not in the old key. Example 2 illustrates a chromatic modulation between C major and the key of A minor; the pitch G# in the third chord is the leading tone of the new key, but is not present in the old key of C major. The introduction of the chromatic tone helps effect the modulation to the new tonal center.

A more complicated chromatic modulation technique involves the use of enharmonic reinterpretations of chords to redirect the harmonic progression to a new, unexpected tonal center. This can be accomplished through the enharmonic respelling of a German augmented sixth chord as [Page 707]a dominant seventh chord; for example, as shown in Example 3a and 3b, or with the enharmonic respelling of a fully diminished seventh chord. Because the chords involved are enharmonically equivalent, the function of the enharmonically respelled chord is understood only in retrospect upon hearing the resolution of the chord.

An even more abrupt form of modulation is the direct modulation, where there is no transition from one tonal center to another; in its typical use, a phrase ends with a cadence in one key and the subsequent phrase will begin immediately in the new key.

Modulations can also be effected through the use of harmonic sequences; these can take many different forms and can be diatonically, chromatically, or enharmonically accomplished.

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