Imitation and sequence are the two most powerful techniques for developing melodic material in polyphonic music. Together they explain how composers create minutes of music from a few measures of initial material.
Imitation: The Echo Effect
Imitation is the technique of immediately restating a melodic phrase in another voice, typically before the first voice has finished. It is distinguished from strict canon (where the imitation continues for the entire length of the piece) by being local — the imitating voice eventually diverges into its own material. Imitation is the primary texture-building technique in Renaissance polyphony and Baroque counterpoint: virtually every mass movement by Palestrina, every chorale prelude by Bach, and every invention and sinfonia begins with imitation.
Imitation can be classified along several axes:
- Real imitation reproduces the original phrase with exact intervals and rhythms at a different pitch level. The transposition is mathematically strict.
- Tonal imitation modifies one or two notes of the transposed phrase to preserve diatonic content, as in fugal answers. The transposition is musically adjusted to fit the key.
- Imitation at the unison or octave is simplest. The imitating voice begins on the same pitch class as the leader, just delayed in time. The Bach two-part inventions frequently open with octave imitation.
- Imitation at the fifth is the basis of fugue. The imitating voice begins a fifth higher (or fourth lower), creating a tonic-dominant relationship between the leader and follower.
- Imitation at the third or second is possible but rarer and requires careful voice-leading. The resulting harmonic relationship is more dissonant and less stable than fifth-related imitation.
- Imitation by inversion reverses every interval — what went up now goes down. Bach uses this technique extensively in the Art of Fugue.
The aesthetic effect of imitation is the sense of musical dialogue — one voice speaks a phrase, another responds with the same phrase before the first has finished speaking. This creates an illusion of self-replicating life, as if the musical idea is generating copies of itself. It is this quality that gives the music of Josquin, Victoria, Lassus, and Bach its characteristic sense of organic inevitability.
Sequence: Systematic Transposition
A sequence is the immediate repetition of a melodic or harmonic pattern at a different pitch level. In a melodic sequence, the same figure is repeated one step higher or lower: the figure C-E-D might be repeated as D-F-E, then E-G-F, descending or ascending stepwise. In a harmonic sequence, the same chord pattern is transposed: I-V, ii-vi, iii-vii (descending circle of fifths). Sequences are the principal means of harmonic motion in tonal music — they provide directed, purposeful movement through a range of keys or harmonic areas without losing momentum.
Sequences are classified by their pattern of root motion and by whether they preserve the diatonic scale (diatonic sequence) or modulate continuously by literal transposition (chromatic or modulating sequence). The most important diatonic sequences in tonal music are:
- Descending circle of fifths (or falling fifth sequence): bass descends by a fifth at each step. The pattern in C major: C-F-Bdim-Em-Am-Dm-G-C. This is the most common harmonic sequence in tonal music.
- Ascending 5-6 sequence: the bass rises stepwise while each chord alternates between root position and first inversion. Often used in Baroque music for stepwise modulating passages.
- Descending 5-6 sequence: the bass falls stepwise with alternating root position and first inversion. Used by Pachelbel and many other Baroque composers.
- Ascending circle of fifths: rare but appears in specific contexts, often modulating upward.
- Sequence by descending thirds: bass descends by a third at each step. Used for harmonic colour and modulation to flat-side keys.
The Descending Fifth Sequence in Detail
The descending fifth sequence is so common that it deserves special attention. From any chord, the bass descends a fifth (or rises a fourth) to the next chord. In C major: C-F-Bdim-Em-Am-Dm-G-C. This bass motion creates a strong sense of forward momentum because each bass step is a fifth, the most acoustically powerful harmonic interval. The sequence can be embellished with seventh chords (V7 to I7 to IV7 to VII7 to III7 to VI7 to II7 to V7), creating the basis of many jazz standards and Baroque ritornelli.
Vivaldi's concertos are famous for their use of the descending fifth sequence; Bach's preludes and inventions are full of sequential passages. The sequence functions as a kind of harmonic conveyor belt: once started, it propels the music inevitably forward until it is broken by a cadence or by a deflection to a new key.
Sequence and Fugue Episodes
Fugue episodes are typically built on sequences, usually using the interval content of the subject or countersubject in a two-voice sequential pattern. The sequence provides several functions: clear harmonic direction (moving to a new key for the next subject entry); motivic coherence (using the same material as the rest of the fugue); and rhythmic energy (sequences are inherently propulsive because they create an expectation of continuation). The episodes in Bach's fugues are almost always sequential, and identifying the sequential pattern is the first step in episode analysis.
Sequences in Popular Music
Sequences appear constantly in popular and jazz music, often unrecognised as such. The chord progression of Pachelbel's Canon (I-V-vi-iii-IV-I-IV-V) has been used in dozens of pop songs from "Streets of London" to "Don't Look Back in Anger" — it is essentially a descending-third sequence interrupted at the end. Jazz standards use the descending fifth sequence as the foundation of bridges and as the basic harmonic motion of countless tunes (the bridge of "I Got Rhythm" is a four-step descending fifth sequence).
Sources and Further Reading
- Open Music Theory — Sequences (openmusictheory.net, CC BY-SA 4.0).
- Aldwell and Schachter. Harmony and Voice Leading. Schirmer/Cengage. Chapter 11: Sequences.
- Laitz. The Complete Musician. Oxford University Press. Chapter 12.
- Gauldin. A Practical Approach to 18th-Century Counterpoint. Waveland Press.
- Fux/Mann. The Study of Counterpoint. Norton.