What is it? Well, like many other harmonic techniques, it serves to expand an idea. An awful lot of music consists of repeating things in interesting ways and one of the most interesting ways of repeating something is by means of a harmonic sequence. If you repeat a melodic or harmonic idea on one or more different scale degrees, you get a sequence. Here are three of the four basic sequences:
|Click to enlarge|
Much of the time the repetition includes the upper voices as well as the bass. They look pretty dull in their bare-bones outline, but in the hands of a master the sequence can be spectacular. What you can do to dress up a basic sequence is arpeggiate the chords with some fancy figuration. Another thing you can do is change the harmonic rhythm. In the following example by Bach he presents three sequences. The first is a falling 3rds sequence with each repeat taking half a measure. This is followed by another sequence that rises by step (the third iteration is incomplete) with each repeat taking a full measure and finally a third sequence that also rises by step, with chromatic passing notes, with each iteration taking only one beat, but there are six iterations in all. This occurs at the very beginning of the piece, a prelude from the Well-Tempered Clavier and all of this is over in a few seconds!
Here is Glenn Gould playing the Prelude and Fugue #21 from the Well-Tempered Clavier Bk 1:
The three different sequences take only eighteen seconds in that performance! Here is how they look in score:
And that's one of the many reasons why Bach is number one!