Modulation – free music theory course

To modulate is to change key for a while: a new tonic settles in, confirmed by a cadence. It's one of music's great dramatic devices: departure, journey, return.

Closely related keys

The most natural modulations go to closely related keys, whose key signatures differ by at most one accidental. For C major: G major (V), F major (IV), and their relatives A minor (vi), E minor (iii) and D minor (ii). In practice: the keys whose tonic is a major or minor chord of the starting scale.

In classical music, the most frequent modulation is to the dominant in major, and to the relative major in minor.

Pivot-chord modulation

The smoothest technique is to find a chord common to both keys and reinterpret it. Am is vi in C major… but also ii in G major. We arrive on Am in C and leave it in G, heading for the dominant D7 and then the new tonic G.

M:4/4
L:1/4
%%staves {RH LH}
V:RH clef=treble
V:LH clef=bass
K:C
[V:RH] "C"[EGc]2 "F"[FAc]2 | "Am"[EAc]2 "D7"[^FAc]2 | "G"[DGB]4 |]
[V:LH] "_C: I"C,2 "_IV"F,2 | "_vi = ii"A,,2 "_G: V7"D,2 | "_I"G,,4 |]

The F♯ of D7 is the signal: it's the new leading tone. The perfect cadence D7 → G confirms the new key.

Other techniques

Direct (phrase) modulation: the new phrase simply starts in the new key. The famous “truck-driver's gear change” up a whole tone near the end of pop songs is an example.
Common-tone modulation: a single held note acts as a hinge between two distant chords (a favourite of Schubert's).
Chromatic modulation: one voice slides a semitone and transforms the chord.
Diminished-seventh modulation: the symmetrical °7 chord can be reinterpreted as four different leading-tone chords, leading to four different keys.

Modulation or tonicisation? If the new tonic is confirmed by a cadence and lasts (several bars, a phrase), it's a modulation. If it's merely touched by a secondary dominant, it's a tonicisation.

In analysis, the key signature doesn't necessarily change when the music modulates! In a classical sonata, the exposition moves to the dominant using accidentals, with no change of signature. Look for new leading tones and cadences.