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In music, modulation is the change from one tonality (tonic, or tonality) to another. This may or may not be accompanied by a change in key signature (a key change). Modulations articulate or create the structure or musical form of many pieces, as well as add interest. Treatment of a non-tonic chord as a temporary tonic, for less than a phrase, is considered tonicization.
The quasi-tonic is the tonic of the new key established by the modulation. The modulating dominant is the dominant of the quasi-tonic. The pivot chord precedes the modulating dominant, and is common to both the key of the immediately preceding tonic, and the new key of the quasi-tonic. For example, in a modulation to the dominant, ii/V–V/V–V could be a pivot chord, modulating dominant, and quasi-tonic.
Any chord with the same root note and chord quality (major, minor, diminished) can be used as the pivot chord. Therefore, chords that are not generally found in the style of the piece (for example, major VII chords in a J. S. Bach-style chorale) are also not likely to be chosen as the pivot chord. The most common pivot chords are the predominant chords (ii and IV) in the new key. In analysis of a piece that uses this style of modulation, the common chord is labeled with its function in both the original and the destination keys, as it can be heard either way.
Where an altered chord is used as a pivot chord in either the old or new key (or both), this would be referred to as altered common chord modulation, in order to distinguish the chromaticism that would be introduced from the otherwise diatonic method.
By combining the diminished seventh with a dominant seventh and/or augmented sixth, altering only one pivot note (by a half tone), it is possible to modulate quite smoothly from any key to any other in at most three chords, no matter how distant the starting and ending keys (be aware that only when modulating between key signatures featuring double-sharps/flats may the need to respell natural notes enharmonically arise); however, this may or may not require the use of altered chords (operating in the harmonic minor without an augmented sixth would not) where the effect can be less subtle than other modulations. The following are examples used to describe this in chord progressions starting from the key of D minor (these chords may instead be used in other keys as , such as the parallel major, or other forms of the minor):
Note that in standard voice leading practice, any type of augmented sixth chord favors a resolution to the dominant chord (see: augmented sixth chord), with the exception of the German sixth, where it is difficult to avoid incurring parallel fifths; to prevent this, a cadential six four is commonly introduced before the dominant chord (which would then typically resolve to the tonic to establish tonality in the new key), or an Italian/French sixth is used instead.
In short, lowering any note of a diminished seventh chord by a half tone leads to a dominant seventh chord (or German sixth enharmonically), the lowered note being the root of the new chord. Raising any note of a diminished seventh chord by a half tone leads to a half-diminished seventh chord, the root of which is a whole step above the raised note. This means that any diminished chord can be modulated to eight different chords by simply lowering or raising any of its notes. If also employing enharmonic respelling of the diminished seventh chord, such as that beginning the modulation in the above examples (allowing for three other possible diminished seventh chords in other keys), the versatility of this combination technique and the wide range of available options in key modulation become apparent.
This type of modulation is particularly common in Romantic music, in which chromaticism rose to prominence.
Other types of enharmonic modulation include the augmented triad (III+) and French sixth (Fr+6). Augmented triad modulation occurs in the same fashion as the diminished seventh, that is, to modulate to another augmented triad in a key: a major third (M3 as root) or minor sixth (A5 as root) away. French augmented sixth (Fr+6) modulation is achieved similarly but by respelling both notes of either the top or bottom major third (i.e. root and major third or diminished fifth and augmented sixth) enharmonically and inverting with the other major third (i.e. diminished fifth and augmented sixth becomes root and major third of the new Fr+6); either choice results in the same chord and key modulation (a tritone away), as the diminished fifth always becomes the new root.
Starting from a major chord, for example G major (G–B–D), there are twelve potential goals using a common-tone modulation: G minor, G minor, B major, B major, B minor, C major, C minor, D minor, D major, E major, E major, E minor.Kopp, David (2006). Chromatic Transformations in Nineteenth-Century Music, p. 50. Cambridge University Press. . After Marx, Adolph Bernard. Theory and Practice (1837). Trans. Saroni. Thus common-tone modulations are convenient for modulation by diatonic or chromatic third.
In this case, the IV chord in C major (F major) would be spelled F–A–C, the V/ii chord in C major (A major) spelled A–C–E, and the ii chord in C major (D minor), D–F–A. Thus the chromaticism, C–C–D, along the three chords; this could easily be part-written so those notes all occurred in one voice. Despite the common chord (ii in C major or i in D minor), this modulation is chromatic due to this inflection.
The consonant triads for chromatic modulation are III, VI, II, iv, vii, and VII in major, and iii, vi, II, iv, ii, and vii in minor.
In the example pictured, a chromatic modulation from F major to D minor:
In this case, the V chord in F major (C major) would be spelled C–E–G, the V in D minor (A major) would be spelled A–C–E. Thus the chromaticism, C–C–D, which is here split between voices but may often easily be part-written so that all three notes occur in one voice.
The combination of chromatic modulation with enharmonic modulation in late Romantic music led to extremely complex progressions in the music of such composers as César Franck, in which two or three key shifts may occur in the space of a single bar, each phrase ends in a key harmonically remote from its beginning, and great dramatic tension is built while all sense of underlying tonality is temporarily in abeyance. Good examples are to be found in the opening of his Symphony in D minor, of which he himself said (see ) "I dared much, but the next time, you will see, I will dare even more..."; and his Trois Chorals for organ, especially the first and third of these, indeed fulfill that promise.
An unprepared modulation is a modulation "without any harmonic bridge", characteristic of impressionism.Rudolph Reti (1978). Tonality, Atonality, Pantonality. Westport, CT: Greenwood Press. .
For example:
When phrase modulation comes at or near the end of a musical piece, it is referred to as a truck driver's gear change, especially in popular music.
| | D || → | D7 | G7 | C7 | F |
| V7 | V7 | V7 | I |
If a given key were G major, the following chart could be used:
| C | — | G | — | D |
From this, the musician would go to G major's relative minor which is E minor, and potentially to C major and D major's related minor as well (a musician who does not know the related minor for C and D major may also go P5 below or above E minor).
| C | — | G | — | D |
| Am | Em | Bm |
By using the relative minor keys one can find the specific key that the key can modulate into.
Many musicians use the circle of fifths to find these keys and make similar charts to help with the modulation.
Changes of key may also represent changes in mood. In many genres of music, moving from a lower key to a higher often indicates an increase in energy.
Change of key is not possible in the full chromatic or the twelve tone technique, as the modulatory space is completely filled; i.e., if every pitch is equal and ubiquitous there is nowhere else to go. Thus other differentiating methods are used, most importantly ordering and permutation. However, certain pitch formations may be used as a "tonic" or home area.
Modulation may also occur from a single tonality to a polytonality, often by beginning with a duplicated tonic chord and modulating the chords in contrary motion until the desired polytonality is reached.
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