Modulation
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Snip from Wiki . .
Modulation (music)
Example of modulation from the tonic to the dominant.[1] Play (help·info)
Key signature change example: C major to C minor.
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.
— Charles-Henri Blainville (1767)[2]