modulation
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The process of changing from one key to another within a piece.
Modulation is the process of moving from one key to another within a piece of music. A well-crafted modulation uses pivot chords — chords that belong to both the old and new keys — to make the transition sound smooth and natural rather than abrupt.
Modulation is one of the most powerful tools in a composer's arsenal. It can create a sense of journey, heighten drama, or provide contrast between sections. Common modulations move to closely related keys (the dominant or relative minor), while more distant modulations create a striking effect. Schubert was a master of unexpected, magical modulations that shift the harmonic colour in an instant.