To modulate means to move from one key to another. This involves a change in what the listener perceives to be the tonic chord. This means that the qualities of the tonic chord must thus be transferred to a new chord. When this new chord has been established as the tonic, the music is said to have changed key. There are different Types of Modulation. Some methods are suitable for closely related keys—such as diatonic and chromatic modulation—while there are others that are only suitable for distantly related keys, such as abrupt and enharmonic modulation.
The process of modulation has two main features. There is the journey that is taken from one key to another, and there is the way in which we get there. The journey itself concerns the distance traveled between keys. This is to do with the fact that there are some keys that represent a relatively short journey, while others require a more extended or even abrupt journey. In a traditional sense, the distance traveled is measured by how closely related the two keys are. This closeness of relationship is generally determined by how many notes the two keys have in common.
Diatonic modulation is where the change of key is effected through use of chords that belong to both keys. This common chord is thus used as a pivot chord in order to create a bridge to cross over to the new key. Because this type of modulation necessitates that the two keys share some chords in common, it is only really suitable for modulation to the more closely related keys.
Closely Related Keys
Looking at the key of C major at the top of the circular diagram, we can immediately see that the two most closely related major keys are those whose keynotes are a fifth up (G major) and a fifth down (F major). This is because the key of G major requires only one sharp, which means that the other six notes are identical to the C major scale. Similarly, the key of F major requires only one flat. So again, F major has six notes that are identical to the C major scale.
A common chord, meaning a chord that is common to each of two keys, offers a smooth introduction to the new key, since it is diatonic to both the old and the new key. This common chord is often called a pivot chord because it becomes a sort of middle ground between the two keys. Common-chord modulation is the name given to a modulation where a common chord (or chords) exists.
The Relative Major/Minor Connection
The circle of fifths also shows us that there is another important key that is closely related to the key of C major. This is the relative minor key of A. The closeness of the relationship between these two keys can be gauged from the fact that they both use the same set of notes, the difference between them lying in the note that is perceived as the tonic. This key relationship is different than the relationship with F and G major by the fact that it also involves a change of mode. To change key from C major to the relative minor is also to change mode—from major to minor.
Because the key of C major has a close relationship to the relative minor key of A, C major also shares a close relationship with those keys whose tonic notes lie a fifth above and below the key of the relative minor. This is because a fifth above the tonic of the relative minor, we have the tonic of the key of E minor, which, like G major, uses only one sharp. And a fifth below the tonic of A minor, we have the tonic of the key of D minor, which, like F major, only uses one flat. Summarizing these observations then gives us a picture of those keys that can be considered to be most closely related to the key of C. To find the relative minor of any major scale, proceed to the sixth degree of that scale. This tone is the tonic of the relative minor
The Parallel Major/Minor Connection
Another criterion that can be used to demonstrate a link between keys is the strong connection between those keys that share the same tonic note. Therefore, for example, the key of C major uses exactly the same tonic as the key of C minor. On these grounds, there is a common link between the two keys. This link is strengthened by the fact that both keys possess the primary roots of C, F, and G as well as having the same dominant seventh chord.
This link between keys that share the same tonic is called the parallel major/minor link. Therefore, the key of C minor is the parallel minor key to C major, and the key of C major is the parallel major to the key of C minor. However, because a modulation is said to be a change of key, this being qualified as a change of tonal center, then to move between C major and minor is perhaps not a true modulation. Nonetheless, it can be a great way to achieve contrast in our music.
Distantly Related Keys
All keys that are diametrically opposite to one another on the circle of fifths represent a maximum possible distance between keys. Although the two keys are therefore distantly related, this does not prevent a modulation between them. It is just that a modulation of this sort is more radical and tricky, and achieving such a modulation often requires ingenuity on behalf of the composer/songwriter.
The Process of Modulation
The Initial Key
It is not essential that the tonic chord should appear, but the dominant must be made to sound as such.
The Pivot Chord
The pivot chord selected is preferably not the V of the second key. In the second stage we are still at the point where only the composer is aware that a modulation is to take place. The sounding of the dominant chord of the new key belongs to a later stage, when the hearer is made to realize that a new tonal center is being felt. To put it a little more techrocaHy, the pivot chord should be’ placed in advance of the appearance of the dominant chord of the new key.
The New Key
Establishment of the new key is accomplished by means of the cadence which ends the phrase, although there may occur strong progressions in the key before the cadence. The cadence may be either a half cadence or an authentic cadence.
Phrase Modulation (Abrupt Modulation)
Abrupt modulation is a sudden change of key that comes without warning or preparation. As such, abrupt modulation represents the simplest type of modulation that is possible. Phrase modulation, also known as direct modulation, occurs between phrases, periods, or larger sections where a phrase cadences in one key, and the next phrase begins immediately in a different key.
A chromatic modulation occurs at the point of a chromatic progression (a progression that involves the chromatic inflection of one or more tones). Chromatic modulations often occur in passages where the two keys involved are not closely related. They are somewhat less smooth than the common chord modulation and, on occasion, can call attention to the modulation. Chromatic modulation is effected through introducing chromatic alterations of diatonic scale degrees. By chromatic, I mean notes that do not belong to the diatonic scale of the key in question. The letter name remains the same in a chromatic progression.
By including these chromatic notes, along with the regular diatonic notes of the scale of C major, we obtain a complete chromatic scale. Notice, however, that the scale is notated differently depending upon whether it is rising or falling. This type of chromatic scale is called the melodic form of chromatic scale, and for a very good reason. Notice that any chromatic note preceded by a sharp is a rising chromatic note obtained by sharpening the previous note. Rising chromatic notes are notated in this way because they are assumed to be chromatic passing notes that are leading smoothly up to the note that follows. In contrast to this, any chromatic note preceded by a flat is a falling chromatic note obtained by flattening the previous note. And it is notated as a flat because it, too, is assumed to be a chromatic passing note that is falling down to the degree that follows. So the chromatic notes are written in such a way as to reflect their use as passing notes either to the scale degree above them or to the scale degree below them. In this way, a rising chromatic auxiliary would be written as a flat, and a falling chromatic auxiliary would be written as a sharp.
Chromatic modulation is most commonly effected by using these chromatic notes as part of a tritone that acts as a dominant agent of the new key (they are acting as agents of dominant function). To be a true chromatic modulation, the chord progression must not cadence to the new tonic, but also depart from it as a new tonic chord. Otherwise, the modulation is said to be transitory—a brief visit to another key as a means of broadening the horizon of a chord progression.
Enharmonic modulation is used to modulate to remote keys. This type of modulation is termed enharmonic because it largely depends upon the process of the enharmonic respelling of chords such as the dominant seventh, diminished seventh, and augmented triad. These chords are therefore being used as an enharmonic pivot chord between the two keys. Let us first consider the principle behind enharmonic modulation, which is the process of enharmonic respelling. This is based on the idea that the same note—B, for example—can also be written as note Cb. Although in terms of their pitch, notes B and Cb are the same note, in terms of their place in the scheme of key relations, they are very distant from one another. Therefore, while we can find note B in the key of C, in the circle of fifths we only encounter the note Cb when we reach the key of Gb major. And from the standpoint of the scheme of key relations represented by the circle of fifths, the key of Gb major is about as far from the key of C as it is possible to get.
Now that you understand the principle of enharmonic respelling, let us see how and why it applies to certain chords. It applies to the dominant seventh and the diminished seventh chord because they contain the interval of the tritone. The tritone is probably the most tonally unstable musical interval that there is. This instability is used to good effect when it is employed as an agent of dominant tension. Through enharmonically respelling the tritone, we can actually redirect its tension and harness it to a dominant chord from a different key. Let us now see how.
In the key of C major, there is only one tritone, and this uses the notes F and B. As a part of the dominant seventh chord of that key, the tritone tends to resolve in a certain way, the B—leading note—rising up to the tonic note C, and the F resolving stepwise down to the third of the tonic triad, which is E (or Eb in the case of the minor key). However, we could also respell the same tritone as F and Cb. Spelled in this way, we find the same interval being used in the dominant seventh chord of a totally different key—the key of Gb major. By enharmonically respelling the interval in this way, therefore, the triton acquires different implications for resolution. While the tritone F and B resolves outwardly to the sixth E and C—in other words, the tonic chord of C major the tritone Cb and F resolves inwardly to the notes Bb and Gb—that is, the tonic chord of Gb major .
The same tritone therefore effectively offers us a connection with both the keys of C major and Gb major. In this way, through the process of enharmonic respelling, the tritone gives us a subtle doorway that connects keynotes that are diametrically opposite on the circle of fifths. The same tritone therefore effectively offers us a connection with both the keys of C major and Gb major. In this way, through the process of enharmonic respelling, the tritone gives us a subtle doorway that connects keynotes that are diametrically opposite on the circle of fifths.
Wherever we find a chord that has a tritone in it, that tritone can be respelled in order to enter into the territory of a new key. In this sense, the chord that carries the tritone is being used as a pivot that links the two keys. An important chord for the process of enharmonic modulation, therefore, is the diminished seventh chord. This is because the diminished seventh chord consists of numerous equal interlocking tritones. Another chord that is susceptible to enharmonic respelling is the augmented triad on degree three of the harmonic form of the minor scale.
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.
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.
Distant keys may be reached sequentially through closely related keys by chain modulation, for example C to G to D or C to C minor to E♭ major.
Parallel key modulation
A parallel key modulation is a change of mode, but maintains the same tonal center. For example, one section of a composition may be in the key of E major and then modulate to E minor. This can be done directly or facilitated by the various modulation techniques described above. Depending on the length of the modulation and whether or not it returns to the original key, it may or may not be designated by a change of key signature.