TETRACHORD? ISN’T THAT ONE OF THOSE ’80S computer games for the Commodore 64 that’s worth a lot of money on EBay now? Or maybe a tetrachord is the style of nerdy plaid jacket that hunters wear in the woods. Or it could be a body part—I think I had to go to the school nurse once because I got kicked in the tetrachords.
Actually, a tetrachord is a four-note sequence, like half of a scale. Most diatonic scales (which are built using intervals of whole- and half-steps) have seven notes, eight if you count the octave. A tetrachord is a group of four notes, within an interval of a 4th, built in whole- and half-steps. There are two tetrachords in every diatonic scale. Example 1 shows the two tetrachords found in a C major scale.
We hear the advice constantly: practice scales. It’s not bad advice, because mastering scales helps bassists understand melody and harmony in a linear way. Chords are vertical, with several notes sounding at once. Scales are horizontal, with groups of related notes, heard one after the other. Scales and scale fragments are essential building blocks in bass lines and solos. To understand, practice, and apply scales practically, you can break them down into tetrachords.
Example 2 is possibly the world’s most common bass line: a major tetrachord from G, the 5th of the C major scale, up to the root. Example 3 is the beboppish beginning to Sonny Rollins’ “Pent Up House.” You can think of the line as a major tetrachord (D to G) and a minor tetrachord (A to D), separated by a major 2nd (the interval between the notes G and A).
What are the advantages of mastering tetrachords? They’re small, easy-to-grab groups of notes, and many common scales can be created by combining two of the four main tetrachords. Let’s look at the construction of these basic tetrachords.
EXAMPLE NAME WHOLE-/HALF-STEP CONFIGURATION
Ex. 4 Major tetrachord W-W-H
Ex. 5 Minor tetrachord W-H-W
Ex. 6 Phrygian tetrachord H-W-W
Ex. 7 Gypsy or harmonic tetrachord H-m3-H
Example 8 shows a C harmonic minor scale, created by combining a minor tetrachord and a Gypsy tetrachord, which are separated by a whole-step. Example 9 is a C Dorian scale, created by combining two minor tetrachords, separated by a whole-step. Example 10 shows how to create a C Phrygian scale by combining two Phrygian tetrachords, separated by a whole-step.
Experiment with combining tetrachords to create scales. You’ll probably find that thinking in tetrachords presents new ways to hear some of the common scales that you already know. You might even come up with some easy fingerings and cool new sounds, like the diminished scale (Ex. 11). Whereas the C Dorian scale (Ex. 9) is created by combining two minor tetrachords, one whole-step apart (the interval between F and G), the C diminished scale (Ex. 11) is created with two minor tetrachords, one half-step apart (the interval between F and F#).
There are two more four-note groups that I should mention here, even though they do not fit the classical definition of a tetrachord (these four-note groups are not bounded by an interval of a 4th). For our purposes as forward-thinking bassists, let’s stretch the definition and also call these tetrachords, just because they have four notes, and are so darn useful.
EXAMPLE NAME WHOLE-/HALF-STEP CONFIGURATION
Example 12 Diminished tetrachord H-W-H
Example 13 Whole-tone tetrachord W-W-W
Experiment with combining these four-note groups to create scales. Example 14 shows a C Lydian scale, created with a whole-tone tetrachord and a major tetrachord, separated by a half-step. Example 15 is the diminished whole-tone scale (also called the altered scale), created by combining a diminished tetrachord and a whole-tone tetrachord, which are separated by a whole-step.
To practice tetrachords, pick any two and combine them, playing one tetrachord after the other. Figure out which scale you’re playing, and then improvise on just those two tetrachords for a few minutes, until the sound sticks in your ears. By understanding the construction of a scale using tetrachords, you’ll have a deeper understanding of how to use the sound in a musical way. Next time, we’ll look at more practical uses for tetrachords.
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