What are tetrachords? They are tetra, and chord. 4 notes, but they aren't chords. So what are they?
This was a method of using scales that was used a very long time ago. As far as I am aware this predates chords in its usage. They are scales broken up into 4 note groups, hence tetra-. If we were to look at it in C major (because I'm lazy) the first group would be C, D, E, F. This is called the lower tetrachord, and the last group: G, A, B, C. This is the upper tetrachord. Check out how the upper tetrachord of C major is G major, interesting circle of fifths cycle happening with this.
That is all good and well but how should we play this? Pick up your guitar and find C on 3rd fret A, now play the scale up to the fourth(F), then back down to the root, then keep descending down the scale, to the low G on the E string 3rd fret. Then back up to the root. See how the C sounded now leading up from the upper tetrachord? This is the beginning of voice leading.
So apart from that I have an example of playing with tetrachordal ideas:
This was my first improv when I learnt about this. Not exactly what I described above but it's not isolated.
This topic gets interesting when you hit the minor scale. The minor scale is different to modal minor(Aeolian) When playing the minor scale you ascend up the lower tetrachord (1, 2, b3, 4), back down to root, then down the upper tetrachord (1- b7, b6, 5). The interesting bit is when you ascend the upper tetrachord to the root you actually sharpen the b6 and b7 to the major 6 and 7. This approaches the root note much better than the b6 and b7 because the whalf step between the root and the major 7 is a tension building tritone.
And this is where we see our Harmonic Minor scale came into being. It still has the b3 and the b6 but it has a major 7th, which is that half step behind the root that sounds so good when it resolves ascending to the root. So try that now:
1, 2, b3, 4 (C, D, Eb, F) ascend and back to the root
1, 7, b6, 5 (C, B, Ab, G) descend and back to the root
The upper tetra chord was pretty dark and sinister while the lower was the usual minor sound we are used to. The harmonic minor has a really cool place in music but that is off topic from this for now.
Where do we go from here? - Modes
That's right. I am beating this already very bruised dead horse. By phenomena you can link the modes to each note of the major scale but that is the wrong way to look at them. They all have their own distinct flavors. So here are the notes in intervals relative to the major scale and grouped by tonality:
Lydian: 1, 2, 3, #4, 5, 6, 7
Major(Ionian): 1, 2, 3, 4, 5, 6, 7
Myxolydian: 1, 2, 3, 4, 5, 6, b7
Dorian: 1, 2, b3, 4, 5, 6, b7
Aeolian: 1, 2, b3, 4, 5, b6, b7
Phrygian: 1, b2, b3, 4, 5, b6, b7
And let's leave Locrian out of it because it's the odd one out.
The important thing is to think of them as independent of one another. They are their own scale for you to explore the tetrachords of. No note changes for harmonic interest or tonal push. Just play the lower tetrachord up, then down, down upper tetrachord and back up to root. Notice how it sounds at each point where you turn it around.
After this exhausting read I hope you have learnt something. I did my best to stay on the topic of tetrachords but the voice leading of the minor scale was important to understand so you could hear the point of tetrachords better. Any questions or suggestions as always are welcome and appreciated (even constructive criticism, so long as you don't make me feel too bad ;) ) If you guys want a follow up I will do one regarding chords in line with this topic. Don't throw away your old scale licks and arpeggios, this is not the be all and end all way of using chords ;)
Until next time, have fun