Key change

In music, modulation is most commonly the act or process of changing from one key (tonic, or tonal center) 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.

Modulation is the essential part of the art. Without it there is little music, for a piece derives its true beauty not from the large number of fixed modes which it embraces but rather from the subtle fabric of its modulation.

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.

Common-chord modulation (also known as diatonic-pivot-chord modulation) moves from the original key to the destination key (usually a closely related 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 share 4 chords in common: G, B minor, D, E minor. This can be easily determined by a chart similar to the one below, which compares chord 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 (major/minor/diminished) can be used as the "pivot chord." However, chords that are not generally found in the style of the piece (for example, major VII chords in a 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.

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