What an interval actually is

An interval is the distance between two notes, measured in semitones (frets on a guitar). That's it. Every concept in harmony — chords, scales, modes, voice leading, tension and release — is just intervals stacked in different ways.

Each interval has a sound that's recognizable even out of context, like accents. Once you've internalized the twelve intervals inside an octave, you can name what you hear and reverse-engineer almost any melody.

The 13 intervals inside one octave

Here's the full table. The 'semitone' column is also the number of frets between the two notes on one string. The mnemonic column gives you a song to hum whenever you need to recall the sound.

Consonance and dissonance — a hierarchy

Some intervals sound restful (consonant); others sound tense and want to resolve (dissonant). Roughly, from most consonant to most dissonant: octave / unison → P5 → P4 → M3 / m3 → M6 / m6 → m7 / M7 → m2 / M2 → tritone. The consonant ones come from simpler frequency ratios; the dissonant ones from messier ones.

Dissonance isn't 'bad'. It's the engine of motion. A tritone in a dominant 7 chord is what makes V → I sound inevitable. A song with no dissonance is a song that never moves anywhere.

On the fretboard — the shortcuts

• Same string: count frets. 5 frets = P4, 7 = P5, 12 = octave.
• Adjacent strings (except G→B): the next string up is tuned a P4 higher. So the note one string up and two frets back is a M3 higher; same fret one string up is a P4.
• G→B is tuned a M3, not a P4 — that's the source of every 'why doesn't that shape work?' on the B and high E strings.
• Octave shapes: same fret two strings up and two frets up (skipping a string in between). E on fret 7 of the A string → same fret 7 string G is a m7; jump up two strings and two frets to fret 9 on the high E and you're on E again, one octave up.

Inverted intervals — the rule of 9

Flip an interval upside down (move the lower note up an octave) and it becomes its inversion. The two numbers always add to 9: a 3rd inverts to a 6th, a 4th to a 5th, a 2nd to a 7th. Quality flips too: major becomes minor, perfect stays perfect, augmented becomes diminished.

Why this matters: it cuts your memorization in half. Hear a m6 and you can think of it as an inverted M3 — same notes, different bass. This is the secret behind chord inversions and drop voicings.

Practice this week

• Sing each of the 12 intervals up from a single reference pitch (use the low E open string). Check yourself by playing fret 0 then fret X.
• On the A string, find every perfect 5th from the root (fret 0 = A, fret 7 = E above). Now do P5s on the low E string. Now jump strings — A on fret 5 of low E, P5 is E on fret 7 of the A string. That cross-string shape is the bones of every power chord and barre chord.
• Pick a melody you know. Identify the first two intervals by ear before checking the tab.