Using Tetrachords | Hub Guitar

Using Tetrachords

Why Study Tetrachords?

While complete scales allow us to see the full harmonic picture, chords and arpeggios allow us to think in terms of harmony. Tetrachords fall somewhere in the middle, allowing us to see and visualize melodic fragments while zooming in closer than the perspective of a scale.

Original Definition of a Tetrachord

The classical definition of a tetrachord is a four-note scale fragment, spanning a fourth. Stacking two of these scale fragments will result in a scale of some sort. In other words, a typical scale can be divided into two parts: an upper-half tetrachord and a lower-half tetrachord.

Sometimes in real world applications of scales, thinking in terms of an entire scale is counterproductive. Imagine that you are improvising over the following chord progression.

Imaj7VI-7♭VII7V7
Cmaj7A-7B♭7G7

This progression consists of three normal chords that happen in C major. If it weren’t for the B♭7 chord, you could play a C major scale over the whole thing. But because the ♭VII chord shows up and introduces some tomfoolery to the chord progression, we have to adjust our notes. Instead of thinking of an entire chord scale for the ♭VII chord, why don’t we just come up with half of one? After all, the chord lasts only four beats, and we probably aren’t going to play an entire scale within that time.

Since we often don’t need entire scales to play over chords, it is useful to think instead of melodic fragments drawn from the scales.

The Primary Tetrachords

These tetrachords span a perfect fourth, and are found in the diatonic scale.

NameContents
Major/dominant tetrachord1, 2, 3, 4
C, D, E, F
Minor/dorian tetrachord1, 2, ♭3, 4
C, D, E♭, F
Phrygian/locrian tetrachord1, ♭2, ♭3, 4
C, D♭ , E♭, F

Extended Definition Tetrachords

These tetrachords do not span a perfect fourth, or are not found in diatonic harmony

NameContents
Lydian/whole tone tetrachord1, 2, 3, ♯4
C, D, E, F♯
Diminished Tetrachord1, 2, ♭3, ♯4
C, D, E♭, F♯
Gypsy Tetrachord1, ♭2, 3, 4
C, D♭, E, F

Dividing Scales into Tetrachords

Let’s look at some common chord scales and examine how they might be understood if reckoned to be constructed from two separate tetrachords.

Diatonic Mode Construction

Scale Name Upper Tetrachord Lower Tetrachord
C Major Scale C major tetrachord G major tetrachord
C Dorian Scale C minor tetrachord G minor tetrachord
C Phrygian Scale C Phrygian tetrachord G Phrygian tetrachord
C Lydian Scale C Lydian tetrachord G Major tetrachord
C Mixolydian Scale C major tetrachord G minor tetrachord
C Aeolian Scale C minor tetrachord G phrygian tetrachord
C Locrian Scale C locrian tetrachord G♭ lydian tetrachord

Minor Scale Construction

Scale Name Upper Tetrachord Lower Tetrachord
C Minor Scale C minor tetrachord G phrygian tetrachord
C Melodic Minor Scale C minor tetrachord G major tetrachord
C Harmonic Minor scale C minor tetrachord G gypsy tetrachord

Modes of the Melodic and Harmonic Minor

Since the melodic minor scale and harmonic minor scale can be constructed from tetrachords, so can each of their modes.

For example an F Lydian ♭7 scale (the fourth mode of the melodic minor) can be constructed from an F lydian tetrachord and a C minor tetrachord.

Other Scales of Interest

Some scales, such as whole tone and diminished scales, don’t fit neatly into this paradigm. However, you can of course find tetrachords that fit those scales, too. For example, the C whole tone scale could be thought of as a Lydian tetrachord from any of the scale notes: for example, C lydian tetrachord, D lydian tetrachord, E lydian tetrachord, and the like.

An Example

Returning to the previous chord progression, we can come up with tetrachords ideally suited for melodic fragments fitting each chord.

C Major Chord

For C major, the C major tetrachord itself has the 4th of the scale (or tension 11), so we'll choose a G major tetrachord, whose tones are the 5th, 13th, 7th and root: G, A, B, C.

G major tetrachord on guitar.

A Minor Chord

For the A minor chord, the A minor tetrachord will work fine. That gives us the root, 9th, 3rd and 11th: A, B, C, D.

A minor tetrachord on guitar.

B♭ Dominant 7th

Here’s where things get interesting. To choose a tetrachord, first we'd have to assign a chord scale to this chord. That would imply knowing where the chord originated. It seems to be borrowed from C minor, so let’s assume it’s just a plain Mixolydian chord scale. Now we can use B♭ major tetrachord or F minor tetrachord. Let’s use the F minor tetrachord so we don’t have the 11th in the melodic fragment.

F minor tetrachord on guitar.

G Dominant 7th

Since G7 is the V7 chord of the key, let’s use the same logic we used for the previous chord: a D minor tetrachord, avoiding the 11th of the chord.

D minor tetrachord on guitar.

Put It All Together

Chord Tetrachord Melody Notes
Cmaj7 G major G, A, B, C
A-7 A minor A, B, C, D
B♭7 F minor F, G, A♭, B♭
G7 D minor D, E, F, G

Put it Into Action

Here is a brief example of the concept in action: a melody made up of these tetrachords.

Perhaps a better demo of this concept is to see how easy it is to think about four notes per chord instead of seven or eight.

Key Tasks

  • Choose another chord progression. For each chord, choose a fitting tetrachord to play.
  • Improvise using the tetrachords you discovered.

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