What is a Diminished 7th Chord?
The fully diminished 7th chord, or just diminished 7th, is another diminished chord you may encounter. It is very common in jazz and Classical music; it is less common in rock, blues, R&B, and other styles. It does occur enough that you should know it, however.
The diminished 7th is a very dissonant chord. Dissonant chords are rarely played for long stretches of time by themselves. Diminished 7th chords function most as transitionary chords. The tense sound created by them have a tendency to move the music forward by resolving to another chord.
Diminished Seventh Chord Construction
The construction of a diminished 7th chord is unusual. In terms of its thirds construction, it is completely formed from minor thirds stacked on one another—root to 3rd (minor third), 3rd to 5th (minor third) and 5th to 7th (minor third). If you're interested to know, when a chord or scale has a repeating pattern of identical intervals, it is called a symmetrical chord or scale.
Diminished 7th chords contain a new type of 7th we have yet to encounter. So far we've discussed two types of 7ths: the plain old 7th (i.e., the major 7th interval found in a major seventh chord), and we've discussed the flatted 7th (the minor 7th interval found in dominant 7th chords and minor 7th chords). The diminished 7th chord contains a third type of 7th called a double-flat 7th, or diminished 7th interval.
To understand the double-flat 7th, consider the minor 7(♭5) chord (also called the. half-diminished chord). It contained a flatted 3rd, flatted 5th and flatted 7th. In order to get the 7th of the fully-diminished 7th chord, we must flat the 7th again. Therefore, the fully-diminished 7th chord contains a double-flatted seventh. That doesn't mean the double-flat 7 must be spelled with two flats. It only means it is two half-steps below a normal, major 7th. The correct interval name for a double-flatted seventh is a diminished seventh interval. It looks and sounds just like a major sixth interval, but to be theoretically correct, we must call it a diminished 7th interval.
Double flats (and their counterparts, double sharps) can be confusing at first and will seem illogical. If a major 7th is the note B, a minor 7th is called B♭ since it is one note lower. Lowering the 7th again would give us B♭♭ (B double-flat). B double-flat is the very same note as A. So why not call it A? Well, you can and you would get the same result sound-wise, but theoretically an A is a 6th. The seventh chord needs a 7th—in this example, some sort of B. So, we end up with a B♭♭ instead of A even though A is a simpler spelling. It's important to understand because you may encounter it in written music. But, using the simpler spelling probably won't lead to any fist fights on the bandstand. Many times, in written music, the simpler spelling is used to make it easier to read.
Only Three Diminished Chords
Another interesting thing about diminished 7th chords is there are only 3 of them total. Most chords have 12—one unique chord built on each of the 12 notes. If you examine the notes of any particular diminished 7th chord, you will notice any note could be the root and you'd still get the same 4 notes.
For example, Cdim7 (C, E♭, G♭, B♭♭) is the same as an E♭dim7 (E♭, G♭, B♭♭, C), as well as G♭dim7 (G♭, B♭♭, C, E♭) and B♭♭dim7 (B♭♭, C, E♭, G♭). Naturally you have to adjust the spelling for the different chords. You would probably say Adim7, not B♭♭dim7.
So, while you can name diminished 7th chords with any of the 12 notes, there are only 3 four-note sets.
Diminished 7th Chord Pattern Fingering
You'll find diminished 7th chord patterns are fun to play because your hand must spider across the fretboard. It's very easy to pick up the pattern because every note is a minor 3rd apart.
Root (1st finger)
♭3 (4th finger)
♭5th (1st finger)
♭♭7th (4th finger)
Octave root (2nd, or 1st finger)
Another fingering/shape for the diminished 7th chord begins on your 3rd finger. This fingering contains the exact same notes as the previous fingering. The notes are located in different spots on the fretboard, however.
Root (3rd finger)
♭3 (1st finger)
♭5th (4th finger)
♭♭7th (1st finger)
Octave root (4th finger)
Diminished Seventh Chord Symbols
The chord symbols for a diminished 7th chord are pretty simple. The symbol for a diminished triad was a degree symbol (o). For the diminished 7th, we simply add a 7 to it: o7. Or, another common symbol is dim7. Dim7 is probably more clear and easier to read.
A diminished 7th chord built on the root note C would be notated Co7, or Cdim7. Built on an F♯ root, it would be F♯o7, or F♯dim7.
Applying the Diminished Seventh Chord
While this chord doesn't occur too often, if you don't know it, it is very easy to ruin the sound of it and its effect in the music. It almost always functions as a transition chord linking one chord to another. Often the notes of the diminished 7th chord connect chromatically to the notes of the next chord.
Here are a few examples applying the diminished 7th chord pattern.
|Diminished 7th Chord Details|
|Thirds construction:||m3, m3, m3|
|Intervallic construction:||Root, m3, D5, D7, (P8)|
|C diminished 7th spelling:||C, E♭, G♭, B♭♭|