Augmented and diminished intervals

Beyond major, minor and perfect, two more qualities exist. They describe intervals stretched or squashed by a semitone.

• Augmented — a semitone wider than perfect or major.
• Diminished — a semitone narrower than perfect or minor.

Note the asymmetry: diminished is one semitone below minor, so a diminished third is two semitones smaller than a major third.

The tritone and its two names

Six semitones splits the octave exactly in half. That interval has two correct names depending on how it is spelled:

• C to F♯ spans four letter names (C D E F) — an augmented fourth.
• C to G♭ spans five (C D E F G) — a diminished fifth.

Identical in sound, different on paper, and — this is the part that matters — they resolve in opposite directions. An augmented fourth tends to expand outwards to a sixth; a diminished fifth tends to contract inwards to a third.

Both are called the tritone, because the interval spans three whole tones.

Where you actually meet them

In the dominant seventh chord. G7 contains B and F — a diminished fifth. That tritone is what makes the chord restless and gives it its pull towards C.

In the harmonic minor scale. Raising the seventh degree creates an augmented second between degrees six and seven. In A harmonic minor that is F to G♯: three semitones spanning two letter names.

In the diminished and augmented triads. A diminished triad has a diminished fifth; an augmented triad has an augmented fifth. See major vs minor chords.

Spelling decides the name

This is the recurring point. Six semitones is six semitones, but whether you write F♯ or G♭ changes the interval's name, its function, and how a reader expects it to resolve.

In a piece in G major, F♯ is the leading tone and belongs there. Writing G♭ would be technically the same pitch and would misrepresent what the note is doing. See enharmonic notes.