The tritone
The tritone is the distance of six frets: exactly half an octave.
The name means three tones, three whole steps, like F to G to A to B. You’ll also see it called an augmented fourth or a diminished fifth. It’s the same six frets either way, just named from a different note.
It has been called the devil in music for at least three hundred years. The story that the church banned it is a myth; old theory books simply told singers to avoid it, because it was hard to sing and wouldn’t settle.
That refusal to settle is what this page is about. It’s the one interval in a key that never sounds finished, and it’s behind much of the pull you hear in chord progressions, the feeling that a chord has somewhere to go.
Why it’s unstable
Start with what it isn’t: the harshest interval. Two notes one fret apart grind far more than a tritone does. The tritone isn’t ugly. It’s undecided.
Put the twelve notes of the octave on a circle. Any two notes split it into two arcs: the way up from one to the other, and the way back round. C up to G is seven frets. G on up to the next C is five. Seven one way, five the other: a fifth one way round and a fourth the other, two different intervals.
Every pair of notes works like that, with a long way round and a short one. A major third flips into a minor sixth. A minor second flips into a major seventh.
Except one. Six frets up from C is F♯, and six more frets bring you back to C. Both arcs are the same length, so the pair looks exactly the same from either end. Neither note has a better claim to be the bottom one, the note the other sits on.
That’s why a tritone can’t settle on its own. To sound finished, music needs one note that’s clearly home, and a tritone offers two equal candidates. Something has to move.
In a key, something is always ready to. Each of its two notes sits one fret from a note that can settle.
In a major key
A major key has seven notes, which can be paired up twenty-one different ways. Only one of those pairs is six frets apart: the fourth note and the seventh. In C major, that’s F and B.
They aren’t a random pair. Stack the key’s notes in fifths, F C G D A E B, and F and B are the two ends of the chain, six fifths apart. No two notes in the key are further apart than that.
Step 1 of 4. C major, from C to the C above. The numbers between the dots count the frets from one note to the next.
Played together, the pair pulls two ways at once, and the six-fret gap closes to four. If F is the lower of the two, they move apart instead and end up eight frets apart, E below C. Same two landing notes either way.
The pair needs a chord to sound in, and that chord is G7: G, B, D and F. A plain G chord holds only B, one lean. The 7 adds F, the second one. That’s what the 7 in a dominant seventh chord is for. Bdim, the chord built on the seventh note, holds the same pair; it’s G7 without the G.
- High E string: G, the root at fret 3
- B string: F, the seventh at fret 6
- G string: B, the third at fret 4
- High E string: G, the fifth at fret 3
- B string: E, the third at fret 5
- G string: C, the root at fret 5
Step 1 of 4. G7 on the top three strings, from the bottom: B, F and G, at the 4th, 6th and 3rd frets.
Fret numbers
| Chord | G string | B string | High E |
|---|---|---|---|
| G7 V7 | 4 | 6 | 3 |
| C I | 5 | 5 | 3 |
Every major key has exactly one pair: always its fourth and seventh notes, always inside the dominant seventh chord on its fifth note.
| Key | Fourth | Seventh | Lives in |
|---|---|---|---|
| C major | F | B | G7 |
| D♭ major | G♭ | C | A♭7 |
| D major | G | C♯ | A7 |
| E♭ major | A♭ | D | B♭7 |
| E major | A | D♯ | B7 |
| F major | B♭ | E | C7 |
| F♯ major | B | E♯ | C♯7 |
| G major | C | F♯ | D7 |
| A♭ major | D♭ | G | E♭7 |
| A major | D | G♯ | E7 |
| B♭ major | E♭ | A | F7 |
| B major | E | A♯ | F♯7 |
In minor keys
A natural minor has exactly the same notes as C major, so it has the same tritone, B and F, and it resolves the same way.
But C and E, where the pair lands, mean something different here. In C major they were the root and the third. In A minor they’re the third and the fifth. The pull goes straight past A. And the chord that holds the pair is still G7, which is C’s dominant, not A’s. Natural minor’s only tritone is pointing at C, which is why the key tends to drift toward its relative major instead of landing.
Step 1 of 4. Three A minor scales, one to a row. Harmonic minor raises G to G♯; melodic minor raises F to F♯ as well.
D and G♯ both live in E7: E, G♯, B and D. That’s the minor key’s own dominant chord, and it’s why minor-key songs so often use E7, or plain E, where the scale on its own would give you Em. Em has G, not G♯, so nothing in it pulls toward A.
Harmonic minor keeps B and F too, so it has two tritones at once, one pointing at C and one at A. That’s a big part of why it sounds so tense.
- High E string: G♯, the third at fret 4
- B string: E, the root at fret 5
- G string: D, the seventh at fret 7
- High E string: A, the root at fret 5
- B string: E, the fifth at fret 5
- G string: C, the third at fret 5
Step 1 of 4. E7 on the top three strings, from the bottom: D, E and G♯, at the 7th, 5th and 4th frets.
Fret numbers
| Chord | G string | B string | High E |
|---|---|---|---|
| E7 V7 | 7 | 5 | 4 |
| Am i | 5 | 5 | 5 |
Scales without one
Take C major’s tritone away, the F and the B, and five notes are left: C, D, E, G, A. That’s C major pentatonic. A minor pentatonic is the same five notes counted from A. Both pentatonic scales are what’s left of a key once its tritone is gone.
Removing that pair takes more than the tritone with it. F and B were also the notes sitting a fret from their neighbours, the ones that lean. With them gone, no note in the scale points anywhere.
That’s the trade. A pentatonic scale can’t clash, but it can’t land on its own either. Home is wherever the chord underneath says it is, which is how the same five notes can be C major pentatonic over a C chord and A minor pentatonic over Am.
Count the straight lines across each circle and the scales on this site line up neatly: none in the pentatonics, one in major and natural minor, two in harmonic and melodic minor.
The blues scale is minor pentatonic with one note added: in C, it’s G♭, six frets above the root. After all that care to remove it, blues puts a tritone straight back, as a note to pass through rather than land on.
At the far end is the whole-tone scale, every note two frets from the next. Six notes, three tritone pairs: every note has a partner directly across the circle. It’s all tension and no home, and it sounds like it, floating and unresolved. It’s the classic sound of a dream sequence.