TETRACHORDS (tetra is Greek for 4) consist of 4 note groups and are the fundamental building blocks of  KEYS.

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A TETRACHORD is constructed from a group of 4 notes separated from first note to last by 5 half-tones intervals (P4).  The resulting tetrachords are separated by 2 half-tones intervals making the 2nd TETRACHORD a Perfect 5th (P5) away from the ROOT note.  Together, the half-tone INTERVALS from both tetrachords and the interval that separates them will equal 12 half-tones.  (5 + 2 + 5 = 12)

The following example is the C Major scale broken into 2 tetrachords.



To illustrate how tetrachords are used to create keys, look at the following graphics.  The first graphic represents all of the natural and sharp keys.  It goes clockwise from the KEY of C, with no sharps or flats, all the way to the KEY of C# which consists of all sharps.

The notes in GREEN boxes are the KEY names.  The notes in RED boxes represent the note being added to create the next KEY.  Notice how the added note is always on the 7th scale degree or 1 letter note less than the previous KEY note.


Can you see the pattern?


Moving counter-clockwise from C on the circle-of-fifths a Perfect 4th (P4) at a time.

The notes in GREEN boxes represent the KEY names, but the newly flattened note is now in a BLUE box and we start on the 4th note (F) of the C MAJOR scale.  Now notice how instead of the 7th note being sharpened, the 4th note (a Perfect 4th away from the ROOT note) becomes flat.


And there they are – all 15 common keys (3 of which are enharmonic)!

And for those that read the section on MODE patterns, you’ll recognize the 221-2221 pattern immediately as being both IONIAN and MAJOR.

I personally feel that learning to play TETRACHORDS is a fantastic way to learn MAJOR scales and KEYS.  They’re only 4 notes, simple to practice and a great workout for your fretting hand!  If you’re not sure about the pattern to play, they are ALL simply the IONIAN MODE of each KEY broken into 2 – 4 note chunks.