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It’s the Devil’s interval! It’s also the interval that lies at the very heart of the blues, which also became known for awhile as the devil’s music. (That probably has more to do with the Robert Johnson Crossroads legend, and maybe his “Me and the Devil Blues”, but I like to see connections even where they may not exist.)
The tritone is also known as the diminished fifth, or the augmented fourth. It is called a tritone because it is the interval that results from stacking three whole tones on top of each other, e.g. C D E F#. It is the most dissonant of any interval that appears in diatonic harmony, but the interval actually appears in a major scale. It is the interval from the fourth to the seventh in a major scale – from F to B in C major, for example.
A tritone is six half steps, or six frets on the guitar.
Different tuning systems have used different ratios for it. In Just Intonation it is 45:32, but this doesn’t perfectly split the octave which means that a just augmented fourth is a different interval than a just diminished fifth, which would be 64:45.
The tritone is the only other interval that equal temperament gets theoretically correct besides the octave. In equal temperament, the frequency ratio of a tritone is the (square root of 2):1. Thus, in equal temperament an augmented fourth and diminished fifth are exactly the same size.
In the Middle Ages, the tritone was called Diablus, or the Devil’s Interval. Musicians were taught not to use it, and some believe that the prohibition was somehow for theological reasons. It’s place as the Devil’s interval took some hold in classical music. Unless people consider that simply to be medieval craziness, just this year a presidential candidate vowed to ban the tritone from music if elected:
Citing Evangelical Faith, Ted Cruz Calls To Ban “Satanic” Tritone
After a sweet introductions, at :57 seconds, this Dance of the Dead by Saint Saens has some very strong Tritones:
Bartok used it as the basis for the third movement of his Contrasts for Violin, Clarinet and Piano (commissioned by Benny Goodman):
Black Sabbath harks back to this tradition with their self titled song:
And it’s become a staple of metal bands, as anyone who has ever been to a Guitar Center should know from people butchering the following:
But its been used for less demonic purposes. It’s the opening notes of Jimi Hendrix’s Purple Haze:
And when Jobim wrote Desifinado, which means “out of tune,” the out of tune melody notes in the first phrase are a descending tritone:
The reason the tritone is so important generally is because of what it does harmonically. As I said before, in any key the fourth and seventh make a tritone. So in C, the tritone is F and B. Now, a G chord is the dominant of a C chord. That means that, in that key, the G chord naturally wants to move to the C, and that is the strongest natural resolution that we hear. The tritone is the most dissonant interval, and it also strongly wants to resolve. And the G chord, with the F added to it, includes the Tritone, and now has two sources of tension to want to get it to C. By doing that, it can arrive home, and relieve the tension of the tritone. That’s what makes the dominant seventh chord so strong. Moreover, to move from the G7 chord to the C chord the notes of the Tritone both move chromatically one half step, and in opposite directions. The B moves up a half step to C, while the F moves down a half step to E. Thus the Tritone slides easily, and most pleasingly into a very consonant major 3rd of the I chord.
The tritone is so crucial to Blues for a few reasons. First, the Blues is based on dominant sevenths which never fully resolve. So, in a basic blues, the tritone is always either present, or at least suggested.
Second, the tritone itself is also called the blue note. The b5 of the Key is always ok to play in the Blues.
Third, the basic blues structure marries chromaticism with the tritone. What I mean by this is that you can play an entire blues accompaniment by playing nothing but a tritone, and the tritones a half step below and above that tritone. I’ve given an example of this two note accompaniment in one of the attachments.
That example comes in two forms, the first in the key of A. The second in the key of Eb. The two note tritones that accompany the examples are exactly the same. The bass notes have changed. The chords in the A example are a tritone apart from the chords in the Eb sample. This works because the notes which make up thirds in the key of A, become sevenths in the key of Eb, and the sevenths in one key become thirds in the other. This is an illustration of b5 chord substitution, which is one of the fundamental bases of modern jazz harmony.
Here, its enough to see that just the two notes of the tritones of these chords are enough to outline a 12 bar blues harmony, and that the three tritones involved are in three successive half steps. Harmonically, that’s all you need to establish a blues feel.
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