The devil’s interval: The history of the tritone and its surprising sensitive side

Illustration: Pedro Lourenco

Many of us probably naturally gravitate towards intervals such as thirds, sixths, sevenths, perfect fourths, and perfect fifths when we write melodies and harmonies.

The tritone may be one of the last intervals that comes to mind, written off as a sound that’s nothing but tense and unpleasant to the ear. However, the tritone is actually anything but forgettable, with a rich history and tons of potential to convey an array of feelings that transcend its one-dimensional reputation. In this article, let’s walk through what a tritone is, its impact across music history, and its surprising sensitive side that isn’t so diabolical.

What is a tritone?

The tritone is a musical interval that’s composed of two notes that are six semitones, or three adjacent whole tones, apart. Within a major scale, there’s only one instance where a tritone is formed diatonically: between the seventh and fourth scale degrees (ex. B and F form a tritone in the key of C major).

tritone-sheet-musicA tritone formed by C and F#

Here’s what the tritone above sounds like:

      

Depending on the context and how it’s notated, a tritone may also be referred to as an augmented fourth or a diminished fifth. A reason why the interval sounds so dissonant is because it’s right in between being a perfect fourth and a perfect fifth, which are two of the most consonant-sounding intervals after unisons and octaves (in the context of western tonal harmony). Over history, the tritone has made quite a reputation for itself based on its unstable nature, with writing existing on the interval that spans many centuries.

The devil’s interval

As far back as the early Middle Ages, tritones were avoided in ecclesiastical singing due to their unnerving quality. That said, the interval’s first explicit disapproval came from Italian music theorist Guido of Arezzo, who intentionally structured the collection of notes in his (then widely-used) hexachordal system to avoid any possibility of a tritone occurring, opting for a B♭ instead of a B♮ to avoid the clash between an F and a B. This distaste for the tritone continued all the way into the 18th century, when a particular title emerged that referred to the sinister sound: diabolus in musica, or “the devil’s interval.”

Though Guido’s hexachordal system had long fallen out of favor by this time (it was extended around the 14th century to accommodate more notes), the tritone was still viewed by many as a sound that ought to be avoided. This belief was observed particularly closely by the Church, which was a significant authority in music considering the fact that many leading composers of the era were writing secular pieces. The Church made their perspective clear: music should be a beautiful and moving experience, and anything ugly or jarring should not be used to praise the lord — certainly not an interval that’s widely associated with the devil’s name.

There are popular stories of singers and composers being punished or even excommunicated for attempting to sneak the devil’s interval into their music. Though it’s an interesting thought to entertain, unfortunately these are largely unfounded myths that have no real documentation. That said, the very existence of such tales is indicative of the tritone’s massive controversial presence in music history.

The devil’s music

In more recent history, the devil’s interval has been embraced by none other than artists who belong to “the devil’s music:” rock n’ roll and metal. Taking a liking for its dark sound, bands like Metallica and Black Sabbath crafted some of their most iconic riffs and melodies around the tritone.

In Metallica’s “Enter Sandman,” the B♭ in the main guitar riff creates tension against the song’s tonal center of E minor:

In Black Sabbath’s “Black Sabbath,” the use of the tritone denies the listener of any feeling of resolution:

These bands (and countless others) used the tritone in a way that would make Gudio of Arezzo shudder: to intentionally add tension and darkness to their music. Up to this point, the interval may seem like a diabolical one-trick pony. However, ending things here would neglect a whole other side of the tritone — one that’s surprisingly delicate and beautiful.

The angel’s interval…?

After exploring the tritone’s history and listening to some of its most popular use cases, it may be hard to imagine it as anything other than the devil’s interval. However, the tritone can actually transform its character entirely with the power of harmony; as an example, let’s take a look at half-diminished seventh chords.

half-diminished-7-sheet-musicA half-diminished seventh chord built on C

Half-diminished seventh chords contain a root, a minor third, a diminished fifth (a.k.a. a tritone), and a minor seventh. This is what the half-diminished seventh chord notated above sounds like:

      

That may still sound pretty dissonant to you, and many others would agree. The chord contains both a tritone (the diminished fifth) and a minor seventh, which aren’t exactly the most consonant intervals out there.

However, what matters the most in music is context – with the right circumstances, we can make a major chord sound melancholy, a minor chord sound triumphant, and even a tritone sound beautiful. Let’s put this idea into action by adding a tritone to the last four bars of one of the most consonant songs out there: “Twinkle, Twinkle, Little Star.”

As a reference point, below is a standard harmonization of our favorite lullaby:

twinkle-twinkle-standard-harmonization

      

Now, let’s take a listen to what happens when we substitute one of our IV chords with a half-diminished seventh chord:

twinkle-twinkle-half-diminished-seventh-harmonization

      

The half-diminished seventh chord that was so dissonant when heard in isolation now sounds beautiful, adding a melancholy feeling that we couldn’t achieve with the IV chord that it replaced. What’s more, the tritone, our so-called devil’s interval, is without doubt the star that makes this chord shine; its non-diatonic color adds surprise and wonder to the otherwise predictable chord progression, and the fleeting dissonance sweetens the satisfaction of our resolution.

Now that we have the basic idea, let’s hear the excerpt in an arrangement that’s more musical than a melody over block chords; we’ll add some simple arpeggios, transpose everything up by two octaves for emotional effect, and throw in a second half-diminished seventh chord for fun:

twinkle-twinkle-half-diminished-seventh-harmonization-02

      

The big picture

What’s important to take away from all of this is that context is everything when it comes to music, and that it can’t be analyzed solely in segmented, instantaneous moments. We would be doing ourselves a disservice if we assumed that tritones can only be dark and dissonant, just because Guido of Arezzo said so centuries ago. That said, at the end of the day they’re definitely great for doing that as well — so if you want to write your own angsty, Halloween-spirited riff, calling upon the devil’s interval may do just the trick.

October 30, 2019

Harrison Shimazu Harrison Shimazu is the editor of the Splice blog and a composer for video games and film.