A tritone substitution is a great way to make a typical I-ii7-V sound more interesting. Here’s how it works.
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The jazz world has been using the tritone substitution for decades. And true, the chord is typically found in jazz genres, but like all chords you can find a use for it in pretty much any style of music. A tritone substitution usually takes the place of a dominant (V7) chord. So it turns this: C Dm7 G7 C into this: C Dm7 Db7 C. The easy explanation for why Db7 (the flat-II) serves as such a good substitute for G7 is that the chords share two notes: F and B. So first, let’s give a listen to both progressions so that we can actually hear the difference. (The links open a new browser window).
Progression with a dominant chord (G7): C Dm7 G7 C
Progression with a tritone substitution (Db7): C Dm7 Db7 C
It’s called a tritone substitution because the root of Db7 is an augmented 4th (nicknamed a tritone) away from G7. The G7 chord uses the notes G-B-D-F. The Db7 uses the notes Db-F-Ab-Cb. In music a Cb note sounds the same as a B. So the two chords share two pitches: F and B.
Those two pitches happen to be crucial parts of what makes a dominant (V) chord want to move to a tonic (I) chord. And since those two pitches exist in Db7, it makes it a great substitute for the dominant chord.
Besides having those two crucial pitches (F and B), a tritone substitution works well because it allows the bass to slide down by semitones from D, through Db, and finally to C. So the tritone substitution works as a great chromatic passing chord.
Having said that, an alternate way of using the tritone substitution is to place the dominant note in the bass. Doing this creates some delicious dissonances: C Dm7 Db7/G C. You get the bass note G clashing with an F as well as the Ab of the Db7 chord. You also get that G against the Db, which is an augmented 4th interval that requires resolving. So it’s a triple dissonance, but can sound really great in certain situations.
There are other ways to use this Db7 chord, ways that don’t specifically require it to replace a dominant chord. For example, a Db7 can act as part of a progression that changes key from C major to Eb major, like this: C Db7 Bb7/D Eb. That progression uses a chromatically rising bass line.
There is one other chord that is similar to the chord we’ve been discussing, called a Neapolitan chord. The Neapolitan chord takes the flat-II (Db-F-Ab), places the F in the bass, and it takes the place of an F chord that moves to G. So it turns this: C F G C into this: C Db/F G C.
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