<P> 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 . </P> <P> 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 . </P>

What's it called when a song changes key