The Diminished 7th Chord

The diminished 7th chord (dim7) is is an interesting chord in that the 7th interval isn't what we normally think of as a seventh. In other seventh chords (Maj7, Min7, Dom7), the 7th interval is either 10 (minor 7th) or 11 (major 7) half steps. The distance from the root to the 7th in a dim7th chord is 9 half steps, which is normally considered a major 6th interval. In the diminished scale, the 7th note of the scale is 9 half steps from the root, and in this case the interval of 9 half steps is considered a "diminished 7th" interval.

The dim7th chord is also somewhat unique in that any of the chord tones can be the root of the chord. For example, a "Cdim7" chord (C,Eb,Gb,A) can also be called "Ebdim7," "Gbdim7," or "Adim7." This is because the chord is built with stacked minor thirds, so every note is the same distance from the note higher or lower. The chord name usually is derived from the bass (lowest) note played in the chord.

The dim7 chord is often used as a passing chord between two chords whose roots are a major second apart.

For example, play:

Cmaj7 (C,E,G,B) to C#dim7 (C#,E,G,Bb) to Dm7 to

D#dim7 (D#,F#,A,C) to Em7 (E,G,B,D)