Tritone Substitution

One of the most natural progressions in music is the down a fifth progression (which we talked about in a previous unit). A diagram of down a fifth progressions is shown below:

One of the most common sections of this down a fifth root movement is the ii7-V7-I progression. An example of the ii7-V7-I progression in C major is shown below:

However, sometimes (especially in jazz pieces), instead of this ii7-V7-I progression, the following is played:

We might think that the Db chord in the above progression is a Neapolitan or Phrygian II chord as it is a bII7 chord. However, it is not a Neapolitan/Phrygian II chord for two reasons:

First, a neapolitan/Phrygian II chord has a subdominant function and therefore resolves to dominant. The Db7 chord in the above progression resolved directly to I, not to V.

Second, the Neapolitan or Phrygian II is usually a triad. This Db7 chord is a major-minor seventh chord, a dominant seventh chord.

In this case, the Db7 is a substitute for the G7 chord; the Db7 chord is functioning as the dominant in this progression. This use the Db7 chord is called a tritone substitution.

Defintion of Tritone substitution

Tritone Substitution: The substitution of a major-minor 7th chord whose root is a tritone away from the dominant for the dominant 7th chord.

The use of the tritone substitution (or tritone sub) allows the bass line move by half step into the tonic chord instead of by a skip of a fifth; it smooths out the bass line.

Why the Tritone Substitution works

It might seem that substituting a chord whose root is a tritone away from a chord would be jarring and sound like a dominant chord. So why does this substitution work?

It works because the the two chords share the key defining interval in a key – the tritone between the 4th and the 7th scale degree.

In the example of the V7 and its tritone substitution in the key of C shown below:

In the key of C, the 4th and 7th scale degree (F and B) constitute the key defining interval of the tritone.

In the G7 chord above, the B and F (the 3rd and 7th of the chord) form a tritone. In the Db7 chord, the 3rd and 7th (the tritone of this dominant seventh chord) are F and Cb – the same (or enharmonic) to the F and B of the G7 chord. Since the tritone stays the same, the substitution works.

The only real difference is the bass line, which, as stated before, moves down by step to the tonic.

Compare the two progressions again (see below):

Analytical indication

In jazz notation, the tritone sub is just indicated by the chord symbol (e.g., in the above example, the Db7). There is no indication for this chord in roman numeral analysis; the indication TT sub can be used as shown above.