We sit in front of the piano keyboard and someone points to seven white notes named ABCDEFG.

They ask us to play any two of these notes together and say which of these intervals are dissonant and which are consonant.

The answers to this question are, of course subjective and very much subject to the context in which they are played, but let us suppose that we agree with convention and that adjacent notes (a step apart, or the interval of a second) are in some sense dissonant and that the interval between the F and the B (a three tone skip, or the interval of a tritone) is dissonant and that all other combinations of two notes played together are consonant (just bear with this assumption for a moment).

Remove the F and the B from ABCDEFG and what remains is a five note or Pentatonic note set or scale ACDEG. And, as we know, the vast majority of blues, rock, folk melodies and much else are built on the consonant pentatonic scale.

Yes, the Pentatonic scale is usually the very first thing that we are introduced to as children as we sit at the piano, but not usually via the white notes of the A minor pentatonic scale ACDEG. So …

Transpose our five white notes up (or down) to the black note Eb and you have the scale in which we play Chopsticks: the five notes of the Eb minor pentatonic scale: Eb Gb Ab Bb Db. Some people think it would frighten the children to call the Chopsticks scale Eb minor Pentatonic. Note that we have tansposed our white notes up (or down) a Tritone. Note also that it doesn’t matter whether you do so up or down as a Tritone interval divides an octave exactly in two.

Transpose our five white notes up a minor third to C and we have CDEGA which is the C Major Pentatonic scale.

As an aside, knowledge of Pentatonic scales: having them “under the fingers” is quite useful for the composer and improvisor, particularly as Pentatonic melodies can be played simultaneously with a variety of chords, and very simple rules (later!) allow the composer and improvisor to control the degree of dissonance of the melodic lines against the prevaling harmony. We digress … just saying!

But let’s switch our attention back to the 2-note Tritone that we extracted from our Heptatonic 7-note scale to give us our 5-note Pentatonic scale because the two notes of the Tritone can be so vital in directing the harmonic motion in a piece of music, harmonic motion being derived largely from setting up a harmonic tension and then resolving it.

Let’s take a typical harmonic cadence.

Cadence: a progression of two or more chords that concludes a piece of music.

And let’s take a particular type of cadence that occurs commonly in many jazz tunes: the ii-V7-I with the Dominant 7th V7 resolving to the tonic I. In the key of C that would be G7 resolving to CMAJ. The dissonant Tritone BF supplies the tension in G7 that needs to be resolved, BF being the 3rd and flatted 7th of the 4-note chord G7. F and B are not chord tones of the tonic CMAJ.

So, when we’re short of what to play when presented with a Dominant 7th chord, we can do a lot worse than reach for the two notes of the Tritone (Degrees 3 and 7 of G7e.g. B and F)

Since the Tritone BF (equally FB) is the “business” part of the Dominant 7th chord, we can play C#7 which will have the same effect; F being the 3rd and B being its flatted 7th. This is an example of the so-called Tritone Substitution of Dominant 7ths. Whilst we get an extra bit of tension from the C# and the G# of C#7, we get a smooth, descending bass line through the chords’ roots, the semitones D C# C instead of skipping around D G C, the roots of the ii-V-I.

What else can we say about the Tritone? Well, as we saw above, it divides an octave exactly into two (e.g. C F# C). Divide that octave into four and we have C D# F# A which is, of course, a Diminished arpeggio or, looking at it another way, two Tritones C F# and D# A set a minor (diminished) 3rd apart.

Let me know if you can think of more neat properties of a Tritone.