Musical sequence seems to be a simple concept, but the term is actually used in different ways, many of which are similar. This post will hopefully lend some clarity to the subject. The kind of sequence I’m describing is defined in the Oxford Companion to Music as:
The more or less exact repetition of a melody at another level, higher or lower. If the repetition is only in the melody, with changed harmony, it is called a melodic sequence, and if the repetition is followed also in the harmony, a harmonic sequence. If the repetition is made without leaving the original key, which necessarily means that some of the intervals become larger or smaller by a semitone, it is called a tonal sequence. If, in order to preserve the exact intervals, the key is changed, the name given is ‘real sequence’. Sequences that are real in some repetitions and tonal in others (in some instances to avoid carrying the modulation too far) are called mixed sequences.
For starters, let’s examine a famous sequence as see what we can learn from it:
One key thing to notice is that there are both melodic and harmonic elements. The melodic pattern is one bar long; each successive bar moves the pattern down by a step. Since it stays within the notes of the key (F Major), it’s a tonal sequence. If it rigidly maintained it’s pattern of whole/half steps, adjusting the harmony accordingly, it would be a real sequence, which would end up something like this, cadencing in a new key:
One can also frequently find modified sequences, in which repetitions are adapted or embellished. This commonly occurs on the third instance of a sequenced figure, in which a change is made to avoid monotony. This is an example from First Loss in Schumann’s Album for the Young (starting at bar 17) [hear on YouTube]:
Harmonically, the sequence features a pattern of root motion. There are different “main types” of harmonic patterns that are commonly found in sequences, and this one uses the “descending fifth” type: starting with D7, it moves around the circle of fifths from D to G to C to F to B♭, and then returns to non-sequence harmonic progression (C 7 is V7, and it moves to a I- IV – I64 – V7 pattern.
Here’s another great example, from the jazz canon [hear on YouTube]”
And another [hear on YouTube]:
Does it have to be just like this?
No, but these are great examples to use as “classic.”
Some sequences can be “melodic only,” meaning the chords don’t change in a sequential manner. Here’s an example from the jazz standard As Time Goes By [hear on YouTube]:
Other sequences can be “harmonic only,” with melody freely appearing along with it but not divided into repeating motives.
What’s the difference between a transposition and a sequence?
This is a good question. When a passage is repeated at a different pitch level, is it necessarily a sequence? For instance, this section from Billy Joel’s Allentown [hear on YouTube]:
This fragment of music includes a segment that is transposed up a fourth, (a “real transposition” — note how the intervals of the melody repeat identically) with the chord pattern moving right along with it (like a ii-V-I pattern in keys of D Major and G Major). A third repetition begins with “Out in Bethlehem”) before departing from the pattern and moving on to continue the phrase.
But are we observing a sequence, or is it just transposition? That’s a tough question. My feelings are that there is a “spirit” of sequence that isn’t present here. A sequence, as I understand it, is something rooted in Baroque music in which the transposed motives and chord patterns contribute to the progress of a phrase, moving the music along a path to cadence. The music of Allentown doesn’t do that – instead it uses transposed gestures as the beginnings of a musical sentence. There can be all sorts of instances in which a figure is transposed immediately without being in the “spirit” of sequence.
This may not concur with common usage of the term, which seems to apply to any transposition. The explanation of sentence on openmusictheory.com even teaches an example of sentence as using a gesture “fragmented into one-measure units whose second half is sequenced one step lower.” This seems to be a colloquial usage that doesn’t accurately reflect the majesty of a true sequence.