A change of key, whether transient, or until the music
   becomes established in the new key; a shifting of the tonality of a
   piece, so that the harmonies all center upon a new keynote or tonic;
   the art of transition out of the original key into one nearly related,
   and so on, it may be, by successive changes, into a key quite remote.
   There are also sudden and unprepared modulations.