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4. Tritone Nexus
By the way, you might have noticed something during the classification we discussed earlier. With the addition of new chords, a new root motion is born, and that is the nexus using a tritone leap.
In the world of prime chords, there was 5▲5▼, which was all about perfect 5th (or perfect 4th). However, with flat-root chords (♭III, ♭VI, ♭VII) and the VII chords introduced, there rises a possibility of encountering scenes where the root moves by a tritone. As the tritone has very unique musical characteristics and features, it’s better to distinguish this as a separate category.
Regarding this connection, let’s simply call it “Tritone Nexus” and abbreviate it as TT.
Usage of TT Connection
Now, let’s explain the characteristics of TT.
Moving by tritone is the farthest movement possible in the 12-tone world — A tritone equals to 6 semitones, which exactly bisects one octave.
All other root movements are reduced to values smaller than 6 semitones — for example, a 5th down from G to C is 7 semitones, but considering it as a 4th above, it can also be seen as a 5-semitone movement.
In any reinterpretation, a tritone remains tritone. From this perspective, it can be argued that the tritone symbolizes the most distant relationship.
Perhaps due to that distance, TT nexus creates a shocking sensation, with a feel of disconnection, as if the musical scene were flipped 180 degrees. Moreover, TT is a nexus that never occurs within prime chords. Therefore, the movement of that root itself is unfamiliar, and as a result, it brings freshness.
Given this background, when using TT, it seems beneficial to set the destination a non-prime chord; for a synergistic effect is achieved by moving to unfamiliar chords with an unfamiliar motion. In this article, I will introduce TT nexuses to non-prime chords that seem to have high utility.
IV-VII Nexus
Not long ago, we introduced the half-diminished VII
7 chord. This chord is typically used to proceed to III chords. Since this chord itself has a romantic feel, when reaching this chord with TT, a highly emotional development is created.
- IIVVII7III7VIm
It goes like this. Starting calmly with IIV, suddenly accelerate to VII7, then adds even more instability with III7, and conclude with VIm. A well-crafted chord progression. This movement is commonly used in ballads as well as in jazz.
♭VII-III Nexus
VII is known for its floating sensation. Connecting it to the III7 is also quite dramatic and can be surprisingly useful.
- IIm7IΔ7VIIΔ7III7sus4III7
Right after the feeling of ethereal VII, suddenly move to the super unstable III7! The tritone root movoment definitely adds shockingness. Among various chord progressions, this connection can be considered top-class in terms of making drama.
II-♭VI Nexus
Advancing to VI with TT is also cool. It particularly fits well when used in a rock context.
- VIm7IIm7VIIΔ7
IIm typically progresses to V, so by going to the chord a half-step above it, the entirely different sound-colored VI, you can create a quite unexpected development.
I-♯IV Nexus
Even with the recently learned 
IV7, TT can be executed. It leaps straight from the secure and solid I.
- IIm9V6IΔ7IV7
Quite a dramatic development, isn’t it? Upon closer inspection, the chord tones of IV7 are kinda similar to VIm, and by replacing a expected tonic-function chord, you can construct an unanticipated narrative that hinders the musical resolution.
VI-♭III Nexus
Connecting from VIm to III creates a chaotic sensation, making the tonality unsettled. Therefore, it may not be suitable for regular songs, but if you want to evoke a strange ambiguous feeling, there is definitely a use for it.
- VImIIIΔ7VImIIIΔ7VImIIIΔ7IIm7Im7
TT nexuses return to the original root when performed twice. In this case, I utilized this characteristic to loop two chords separated by a tritone. Such unconventional progressions can work well in dance music.
Keep it as a “Joker”
Due to the vastly different tonalities of each, the introduction may seem scattered. However, they all share a common type of nexus, namely the TT, and are collectively useful for creating dramatic developments.
Especially the last three seem like an unconventional chord progression that you might not feel inclined to use unless someone says “It’s usable!” But still, there are musical ideas that can only be created with them, and there is significance in all chord progressions.
So, the idea of interpreting the musical ideas of chord progressions from “root movement,” “quality changes,” and the accompanying “changes in chord tones” is applicable even to non-prime chords. There might have been various unusual chord progressions that you wouldn’t imagine, but first and foremost, music is free, and I’d like you to remember that.
Music centered around prime chords is, in essence, a basic format for popular music simply because it is easy to listen to, and we have the right to step out from there without any basis. However, stepping out without any purpose might not result in good music. By using nexus theory to think about musical ideas, you can achieve an “effective departure” from basic songwriting.
Summary
- The application of nexus theory remains the same for non-prime chords.
- III or VI rooted chords with some accidentals are collectively called “chromatic mediants”.
- Connections using TT, where the root moves by a tritone, can create a shocking efect.
