A sequence in music occurs when a given melodic or harmonic passage is immediately repeated at a different pitch level. It is possible for melody or harmony to form a sequence without the other participating.
A sequence can be described according to its direction (ascending or descending in pitch) and its adherence to the diatonic scale -- that is, the sequence is diatonic if the pitches remain within the scale, or chromatic (or non-diatonic) if pitches outside of the diatonic scale are used and especially if all pitches are shifted by exactly the same interval (i.e., they are transposed). The non-diatonic sequence tends to modulate to a new tonality or to cause temporarily tonicisation.
At least two instances of a sequential pattern -- including the original statement -- are required to identify a sequence, and the pattern should be based on several melody notes or at least two successive harmonies (chords). Although stereotypically associated with Baroque music, and especially the music of Antonio Vivaldi, this device is widespread throughout Western music history.
Examples
A well-known popular example of a threefold descending diatonic sequence is found in the refrain from the Christmas carol "Angels We Have Heard on High," as illustrated immediately below. The one-measure melodic motive is shifted downward at the interval of a second, and the harmonic aspect does so likewise by following the circle of fifths:
The following three-fold ascending chromatic (non-diatonic) sequence occurs in the duet of Abubeker and Fatima from Act III of César Cui's opera Prisoner of the Caucasus (compare a similar passage in the famous Rodgers and Hammerstein song "Do-Re-Mi," composed almost exactly 100 years later):
Handel's "For Unto Us a Child is Born" relies heavily on both melodic and harmonic sequencing, as can be seen in the following excerpt. In this vocal reduction, the soprano and alto lines reiterate a florid two-beat melodic motif for three and a half bars in a series of melodic sequences on the word "born." More subtle, though still present, are the underlying harmonic sequences.
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