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Modulations

In music, modulation is a change of key. The change of key is a slide from a secondary dominant ("a hint of another key") to the actual modulation with a "new" tonic. A brief change of key can also be called a deviation. Defining the terms secondary dominant, deviation, and modulation can sometimes be difficult.

In traditional music theory, modulation is divided into subcategories:

  1. Diatonic modulation means moving into another key via a pivot chord. For example, the pivot chord for modulation from C major to D major can be E minor, the second degree in D major and third degree in C major. The pivot chord is often a predominant for the new key.
  2. In chromatic modulation, one of the notes in the first key changes chromatically to be part of the new key.
  3. In enharmonic modulation, some of the notes in a chord are reinterpreted enharmonically; a symmetrical, diminished four-note chord can be a secondary dominant in several keys.
  4. Modulation can also happen abruptly without any linking chord material (phrase, direct, or abrupt modulation).

The different types of modulation are mainly defined by notation: modulation via a secondary dominant is often chromatic as the tones of the scale are altered (however, modulation via a Neapolitan sixth is often considered diatonic). Enharmonic modulation is also defined by notation. Shift into a parallel key (for example, from C major to C minor) is not considered to be a modulation as the tonic does not change.

In diatonic modulation, defining one modulated chord is questionable if several consecutive chords are shared by both keys. The excerpt by Mozart below modulates from D major to A major. While the chords at the end of the excerpt can be analysed in both keys, a traditional analysis in D major is not reasonable because of the V fourth sixth chord. The cadence can still be heard, both as a "closing" authentic cadence (-> I) or an "incomplete" half cadence (-> V). Modulation by ear can therefore appear highly subjective.

Tonicization

The term tonicization (German Tonikalisierung) denotes a phenomenon in which one of the degrees in a key is perceived as the tonic. The term tonicization could generally be used instead of the concept modulation; it covers both certain types of secondary dominants (momentary tonicization) and modulation.

At the beginning of Mozart's string quartet (K 160/II), F7 resolves into B flat major. In other words, B flat becomes tonicized. In bar 2, the progression V7-I is in A flat major, but the main key is not perceived until the beginning of bar 6. According to traditional chord analysis, it is a composition starting with a chain of secondary dominants as the notation shows that it is in A flat major.

W. A. Mozart: String quartet K160 Part 2

When the fourth degree of C major is raised repeatedly, the G major begins to assume its form. Correspondingly, lowering the leading note B to B flat takes the progression towards F major. The more common notes the keys possess, the closer they are and the easier the tonicization. Ever since the Baroque, tonicization, for example a fifth up (in major keys) or to a relative key (in minor keys), has been a commonplace phenomenon. The excerpt below shows the beginning of the development section of Mozart's Piano Sonata KV 545 Part 1; first, G minor is tonicized, then D minor. The arrows point to the notes that are essential to tonicization.

Linkage to a key can alter in a one-part melody as well: