A tetrachord is a combination of four specific tones. “Tetra” means four and “chord” in this case just means: “a collection of tones”.
It’s not a chord in the sense that you use it to harmonize a melody. Blame the old Greeks for the terminology.
The interval formula for a tetrachord is: W W H
This means the first three tones are each a whole-step apart (W), but the distance to the last tone is only a half-step (H).
(Remember that a half-step is simply two notes next to each other, while a whole-step skips a key.)
Why would you want to know about a tetrachord? Well, you don’t really… but it’s fun anyway.
Here’s a trick: By stacking two tetrachords on top of each other, you create a major scale.
So if we begin at C and build a tetrachord, the tones are: C D E F
Then we skip a whole-step and build a second tetrachord starting on G: G A B C
Verify it for yourself: G to A is a whole-step, A to B is the second whole-step, and B to C is the final half-step.
That means a tetrachord is exactly half of a major scale. Which half? It doesn’t matter!
Here’s the fun part: if we build another tetrachord on top of this we get: D E F# G
Did you realize that we have just created the major scale of G?
If we build another tetrachord on top, we get the D major scale:
And so on… You can go clockwise around the Circle of Fifths this way until you get back to where you started.
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