Augmented, diminished, and the tritone

Take a perfect or a major interval and stretch it a half step wider — it becomes augmented. Squeeze a perfect or a minor interval a half step narrower — diminished. Most of the time you can ignore these. One of them you can't: the tritone.

The tritone

F up to B is a 4th (F–G–A–B, four letters). But it's wide — three whole steps, a half step bigger than a perfect 4th. That makes it an augmented 4th, and it has a nickname from the middle ages: diabolus in musica, "the devil in music."

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F up to B: a 4th by number, but three whole tones wide. Play one then the other, then hold them together in your ear. It doesn't want to sit still.

It's the most restless interval in tonal music. The two notes are itching to move — and they resolve by the two of them squeezing inward to a 3rd, or splaying outward to a 6th:

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The tritone F–B, then it resolves: the B pushes up to C, the F leans down to E. The squeeze splays outward to a 6th and releases. This exact motion is what happens at the end of almost every 'we've arrived home' moment in music.

You'll see why it matters so much in The dominant seventh — the tritone is the live wire inside that chord.

Restful and restless — consonance and dissonance

Intervals sort roughly into "can sit still" and "wants to move":

	Intervals	Feel
Perfect consonance	unison, octave, perfect 5th	totally at rest, a little hollow
Imperfect consonance	major & minor 3rds and 6ths	at rest, but warm and full — melodies and harmony are built on these
Dissonance	2nds, 7ths, the tritone	tense — needs to resolve to a consonance

The perfect 4th is the odd one out: consonant when it sits above a bass note in a chord, treated as a dissonance in strict two-part writing. Don't lose sleep over it.

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A 3rd — the sound melody leans on, restful and warm. Then a 2nd — grinding, wants to move. Both are useful; the 2nd is the spice that makes the 3rd taste like arrival.

Try it

Try

On the D and A strings, play F (low 2, D string) and B (1st finger, A string) together, rolled. Hold it — hear the tension. Now move the B up to C (low 2, A string) and the F down to E (1st finger, D string): E and C, a 6th, resolved. Do it a few times. That release is the sound of a cadence, in miniature.

The usual mistake

Hearing "dissonance" as "mistake." Dissonance isn't wrong — it's the engine. Without the tritone straining toward resolution, tonal music would have no pull, no arrivals, no drama. The dissonances are what make the consonances feel like home.