Quick Tips seem to be this blog's equivalent of buses - after waiting ages for one, three come along at once!

In recent posts on jazz blues progressions and pedal tones I referred to a common jazz reharmonisation technique known as tritone substitution. I thought it would help some of you get the most out of those posts if I looked at this device in a little more detail.

Essentially, tritone substitution involves substituting a dominant chord with another dominant rooted a tritone away from the root of the original chord.

Don't panic if that doesn't make sense just yet – all will become clear!

Firstly, let's define a tritone:

  • A tritone is an alternative term describing the interval of an augmented fourth or diminished fifth – for example, the distance between C and F#.
  • This interval is called a tritone because it consists of exactly three tones (or whole-steps).
  • A tritone divides the octave exactly in half – there are three tones between middle C and the F# above it. There are also three tones from that F# to the C above.

Now, let's look at tritone substitution in action. Here is a ii-V-I progression in the key of C major:

Dm7 G7 | Cmaj7 |

The dominant chord in this case is G7. The note of Db is a tritone away from G so here we can substitute a Db7 chord for the original G7:

Dm7 Db7 | Cmaj7 |

Here is another example in the key of Eb major. The ii-V-I progression in this key is:

Fm7 Bb7 | Ebmaj7 |

The dominant chord in this progression is Bb7. E is a tritone away from Bb so we can substitute a dominant chord built on the root of E.

Fm7 E7 | Ebmaj7 |

Notice how the substitution causes a chromatic root motion – the roots of the chords descend by a semitone (half-step) each time, resulting in smooth bass movement in the altered progression.

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Tritone substitution is not just limited to ii-V-I progressions though – in fact you can use it almost anywhere you see a dominant chord. For example, the middle 8 of 'I Got Rhythm' features dominant chords moving around the cycle of fifths. If the tune was in the key of Bb, this progression would be:

D7 | D7 | G7 | G7 | C7 | C7 | F7 | F7 |

You can use tritone substitution on any or all of these chords. A popular choice here is to substitute the G7 and F7 chords to give a descending chromatic bass movement for the whole 8 bars:

D7 | D7 | Db7 | Db7 | C7 | C7 | B7 | B7 |

This gives a very powerful sense of inevitability to the progression which culminates in the next bar when the harmony returns to the tonic of Bb.

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Whilst you can use tritone substitution at any time in the performance of a tune, if you wish to use it in the head, it's important to consider what's happening in the melody.

For example, let's say we're looking at a Bb7 chord and the melody note is a D. D is the third of a Bb7 chord and this is a very consonant sound. If we use tritone substitution and change the Bb7 to E7, the melody note now becomes the seventh of the chord. This is still a consonance so there's no problem.

However, it's a different story if the melody note is Bb. This is the root of the original Bb7 chord and about as consonant as you can get! If we substitute E7 here, the melody note now becomes the b5 of the underlying chord. This will be very dissonant.

This doesn't mean that we should not use the substitution here - in fact changing the chord to an E7b5 may give an unexpected twist here. Sometimes this can breathe new life into a familiar tune - but it can also sound terrible in the wrong context.

In other words, it's important to consider the implications of the substitution for the melody notes and to be aware that we may be creating dissonance where there was originally melodic consonance.

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Now you know how to use tritone substitution, the obvious question are 'How does it work?' and 'Why do the substitute chords have to be rooted on a note a tritone away rather than any other interval?'

The answer lies in the relationship between the third and seventh of dominant chords a tritone apart. In a V-I resolution, the tones in the dominant chord that demand to resolve to the tonic are the third and seventh. In the resolution of a G7 chord (spelled G, B, D, F) to C major (spelled C, E, G), it is the B and F of the G7 that resolve in semitones to C and E – the root and third of the I chord. If you have access to a piano, play only these notes and listen how they contain the primary sound of the V-I resolution.

Now let's compare a G7 chord to its tritone substitute – Db7:

G7 – G, B, D, F
Db7 – Db, F, Ab, Cb

The third and seventh of the G7 chord are B and F respectively. The third and seventh of Db7 are F and Cb. Cb is enharmonically the same note as B so the primary resolution tones are the same in both chords.

  • When G7 resolves to C major, the notes of B and F resolve by semitones to C and E.
  • When Db7 resolves to C major, the notes of F and Cb resolve by semitones to E and C

In other words, G7 and Db7 are interchangeable because the tension notes that want to resolve are the same in both chords.

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At first, the sound of tritone substitution may be a little strange, but with a little practice you'll discover it to be a powerful tool that can add variety to familiar progressions and breathe new life into old tunes. Like most substitutions, if it's overused it can become very irritating, but a well placed tritone sub can keep the audience on its toes.

I hope you've found this useful. Until next time