The repetition of a motive two more times, each time beginning on a different pitch, usually a step wise motion up or down from the starting pitch of the previous occurence.
The sequence is a basic construction of musical coherence used in a variety of circumstances. It encapsulates two crucial aspects of most musical meaning: repetition with variation. The repetition establishes a pattern for recognition. The variation give the pattern movement: it takes you somewhere. As its name implies, a sequence is a meaningfully linked series of motive statements. The linkage is usually a simple step wise movement so that the motive is simply transposed up or down, perhaps with suitable tonal adjustments. The sequence is frequently used to build episodic material in a fugue, the connective tissue between subject entries.
Consider an example from Bach's Well-Tempered Clavier, Book 1, No. 24 in B minor. Here is the basic unit of the sequence, a short figure or motive of little consequence by itself.
The sequence is formed by repeating the unit, each time moving, in this case, a step lower:
While sequences are most obviously found in the episodes of a fugue, many subjects and countersubjects contain embedded sequences which lengthen the melody while also establishing internal coherence and sometimes a pleasing symmetry.
Here is the subject from Bach's Well-Tempered Clavier, Book 1, fugue No. 15. Although there are more than simple step wise adjustments, you could say that the subject comprises three different sequences:
Frequently, a second voice imitates a sequence in the first voice, coming in a bit later as you would when singing a round. This is an example of canon, in this case, a sequential canon or a canonical sequence. Again, from Book 1, No. 24 in B minor:
Here is a sequence that plays a crucial role in Shostakovich's Op. 87 Fugue No. 6 in B-minor:
This is a brilliant fugue with sequences, subtle motive transformations, and canonic imitations.
As you might expect, sequences are classified by their properties into different kinds of sequences. The science and art of composing or analyzing sequences is no small matter though the underlying idea is basic. The sequence is in no way restricted to fugue. It is used throughout classical music. You will find the sequence frequently used in the development section of symphonies, string quartets and instrumental sonatas, regardless of the historical period.
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