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.

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.

Extended Definition Tetrachords

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

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

Minor Scale Construction

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

A Minor Chord

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

B♭ Dominant 7^th

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 11^th in the melodic fragment.

G Dominant 7^th

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 11^th of the chord.

Put It All Together

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

As the creator of Hub Guitar, Grey has compiled hundreds of guitar lessons, written several books, and filmed hundreds of video lessons. He teaches private lessons in his Boston studio, as well as via video chat through TakeLessons.