Modulation (music)


In music, modulation is the change from one tonality to another. This may or may not be accompanied by a change in key signature. Modulations articulate or create the structure or form of many pieces, as well as add interest. Treatment of a chord as the tonic for less than a phrase is considered tonicization.

Requirements

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 is a predominant to the modulating dominant and a chord common to both the keys of the tonic and 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.

Types

Common-chord modulation

modulation moves from the original key to the destination key by way of a chord both keys share: "Most modulations are made smoother by using one or more chords that are common to both keys." For example, G major and D major have four triad chords in common: G major, B minor, D major and E minor. This can be easily determined by a chart similar to the one below, which compares triad qualities. The I chord in G major—a G major chord—is also the IV chord in D major, so I in G major and IV in D major are aligned on the chart.
Any chord with the same root note and chord quality can be used as the pivot chord. Therefore, chords that are not generally found in the style of the piece are also not likely to be chosen as the pivot chord. The most common pivot chords are the predominant chords 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, 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.

Enharmonic modulation

An enharmonic modulation takes place when a chord is treated as if it were spelled enharmonically as a functional chord in the destination key, and then proceeds in the destination key. There are two main types of enharmonic modulations: dominant seventh/augmented sixth, and diminished seventh. Any dominant seventh or German sixth can be reinterpreted as the other by respelling the m7 or A6 chord tone in order to modulate to a key a half-step away ; if the fifth-from-root chord tone of a German sixth is omitted, the result is an Italian sixth. A diminished seventh chord meanwhile, can be respelled in multiple other ways to form a diminished seventh chord in a key a minor third, tritone or major sixth away. Where the dominant seventh is found in all diatonic scales, the diminished seventh is found only in the harmonic scale naturally; an augmented sixth is itself an altered chord, relying on the raised fourth scale degree.
By combining the diminished seventh with a dominant seventh and/or augmented sixth, altering only one pivot note, 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 ; however, this may or may not require the use of altered chords 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 :
  • C–E–G–B, C–E–G–B, F–A–C leads to F major—a relative major modulation ; but exactly the same progression enharmonically C–E–G–B, C–E–G–A, E–G–B–E leads somewhat unexpectedly to E natural/harmonic minor—a half-step modulation.
  • C–E–G–B, A–C–E–G, D–F–A leads to the key of D major—a parallel modulation. Enharmonically: C–E–G–B, A–C–E–F, C–E–G modulates to C minor—a major seventh modulation/half-step descending.
  • C–E–G–B, C–E–G–B ≡ E–G–B–D, A–C–E leads to A major—a minor third and relative modulation.
Note that in standard voice leading practice, any type of augmented sixth chord favors a resolution to the dominant 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, 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, 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, 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 and French sixth. 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 or minor sixth away. French augmented sixth modulation is achieved similarly but by respelling both notes of either the top or bottom major third enharmonically and inverting with the other major third ; either choice results in the same chord and key modulation, as the diminished fifth always becomes the new root.

Common-tone modulation

Common-tone modulation uses a sustained or repeated pitch from the old key as a bridge between it and the new key. Usually, this pitch will be held alone before the music continues in the new key. For example, a held F from a section in B major could be used to transition to F major. This is used, for example, in Schubert's Unfinished Symphony. "If all of the notes in the chord are common to both scales, then we call it a common chord modulation. If only one or two of the notes are common, then we call it common tone modulation."
Starting from a major chord, for example G major, 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. Thus common-tone modulations are convenient for modulation by diatonic or chromatic third.

Chromatic modulation

A chromatic modulation is so named because it occurs at the point of a chromatic progression, one which involves the chromatic inflection of one or more notes whose letter name, thus, remains the same though altered through an accidental. Chromatic modulations are often between keys which are not closely related. A secondary dominant or other chromatically altered chord may be used to lead one voice chromatically up or down on the way to the new key. For example, a chromatic modulation from C major to D minor:
In this case, the IV chord in C major would be spelled F–A–C, the V/ii chord in C major spelled A–C–E, and the ii chord in C major, 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, 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 would be spelled C–E–G, the V in D minor 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 "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.

Phrase modulation

Phrase modulation is a modulation in which one phrase ends with a cadence in the original key, and the next phrase begins in the destination key without any transition material linking the two keys. This type of modulation is frequently done to a closely related key—particularly the dominant or the relative major/minor key.
An unprepared modulation is a modulation "without any harmonic bridge", characteristic of impressionism.
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.

Sequential modulation

"A passage in a given key ending in a cadence might be followed by the same passage transposed to another key," this being known as sequential modulation. Although a sequence does not have to modulate, it is also possible to modulate by way of a sequence. A sequential modulation is also called rosalia. The sequential passage will begin in the home key, and may move either diatonically or chromatically. Harmonic function is generally disregarded in a sequence, or, at least, it is far less important than the sequential motion. For this reason, a sequence may end at a point that suggests a different tonality than the home key, and the composition may continue naturally in that key.