Diminished

The principle

If you make a perfect or minor interval a semitone smaller, it becomes diminished. For the fifth that means six instead of seven semitones. Two minor thirds (3 + 3 semitones) make exactly this diminished fifth. It is the same as the tritone and the reason why the diminished triad sounds tense and restless. It asks for resolution, usually to the tonic.

Where it occurs

In a major key the diminished triad is found on the seventh degree, in C major B°. It contains the leading tone B. The diminished seventh chord consists of four tones, each a minor third from the next, and divides the octave evenly. It therefore has no audible root, every inversion is again a diminished seventh chord, and there are only three different ones. From there it can be resolved into four keys without changing a single note. Historically it arises from the harmonic minor scale: in A minor the seventh degree G♯ gives the four-note chord G♯–B–D–F.

The half-diminished seventh chord (B–D–F–A) has a minor seventh, is not symmetrical and sounds softer. The distinguishing feature is the top third: if it is nine semitones from the root to the seventh, the chord is fully diminished; if it is ten, half-diminished.

Common mistakes

• Confusing it with minor: The diminished triad has two minor thirds, minor has one minor and one major.
• Mixing up diminished and half-diminished seventh chords: Only the fully diminished one is symmetrical.
• Using it too often: The diminished seventh chord is an effect and wears out.