What is a Tritone?

A tritone is an interval of 3 tones (whole steps) or 6 semitones (half steps) that sounds particularly dissonant. It can be either expressed as the interval of an augmented 4th or a diminished 5th depending on the context.

History of the Tritone

Legend has it that the interval was banned in medieval times in sacred singing as it was seen as “The Devil’s Interval” due to its unnerving and sinister sound. It gained the nickname “Diabolus in musica” – the devil in music. Whilst there seems to be very little evidence to support the view that it was actually “outlawed” as an interval, the legends surrounding it have inevitably heightened the interest in the interval over the years.

Playing a Tritone

On a piano keyboard play an F followed by a B and you will have played a tritone.
There are three whole steps (tones) between F and B:

Tritone on Piano

Tritone on Piano

The interval you have just played is an augmented 4th. (It is a 4th because there are 4 note letters (staff positions) between the 2 notes – F-G-A-B).
Augmented 4th
If you reverse, the notes and play a B followed by the F above it, this is again an interval of 3 tones (whole steps) and so is again a tritone:

Tritone B to F piano example

Tritone on Piano B to F

This interval is called a diminished 5th.
(It is a 5th because there are 5 note letters (staff positions) between the 2 notes – B-C-D-E-F).
Diminished 5th

Examples of Tritones

Tritones create dissonance in music and so are a vital tool for composers to understand.
Composers have increasingly used the tritone since the Romantic period.
Not surprisingly, the interval has been used to depict themes of evil and despair. Here are a number of examples of its use:

Dante Sonata by Franz Liszt

“Apres une lecture du Dante: Fantasia quasi Sonata” by Franz Liszt is a very famous example. Liszt uses the tritone in the opening theme to conjur up images of hell and torment. You can hear the unease created by the interval:

Franz Liszt Dante Sonata Tritone Example

Dante Sonata Tritone Example

The symphonic poem “Danse Macabre” by Camille Saint-Saens is another example of the disturbing imagery and emotions that the interval can evoke. In this piece, the piercing tritones played by the violins add a terrifying feel to the music.
The piece starts with quiet, gentle music on the strings before the mood is suddenly disturbedyou can hear this change 23 seconds into the performance by the Netherlands Symphony Orchestra in the following video:

However, not all composers have used the interval to create feelings of “evil’.
Other examples include “Maria” from West Side Story and “Purple Haze” by Jimi Hendrix:

Tritone Substitution

Tritone substitution has been used for a considerable amount of time by composers. It has various levels of complexity.
However, the easiest way to understand the basic principal is to consider a perfect cadence (V-I).

In C major, a perfect cadence would be chord G7 to chord C.
However, it is possible to substitute another chord in for chord V (the G7 chord in C major). The chord that we can substitute in is a tritone interval (3 while steps) away from the G3 whole steps away from G leads us to D flat.
So, we can substitute a D flat chord in for our G chord. This creates a really cool effect!
Have a look/listen to these 3 cadences – the first is V-I and the others use the tritone substitute:
Tritone Substitution

Tritone Substitution

You can hear the wonderful variety this substitution brings to the cadence.