Basics of Diminished Chords on the Guitar
Diminished chords are chords that you never use as anything other than a passing chord. On the guitar they sound very dissonant and unstable by themselves, almost unusable. But when placed between the right chords, they make great transitions. Generally speaking, diminished chords have a jazzy flavor to them, and they appear in styles that emphasize voice leading and dominant functions.
There are three types of diminished chords: Diminished triads, half diminished, and diminished 7th, which is also called a fully diminished chord.
The diminished triad is what naturally occurs on the 7th degree of the major scale. It’s 1-f3-f5. That’s a root, minor 3rd, and flat (or diminished) 5th, three minor 3rds in a row. You can also call this minorf5. In the key of G, the 7th triad is Fs-A-C. That’s Fsdim (Fs”) or Fsmf5. Diminished triads are one of the four types of basic triads: major, minor, diminished, and augmented.
If you add a 7th to the diminished triad in the major scale, then you get 1-f3-f5-f7. In the key of G, this is Fs-A-C-E. That’s three minor 3rds from 1-f3-f5 and then a major 3rd from f5-f7. This is called a half diminished chord for reasons you see in a moment. You can also call this a minor7f5.
A diminished 7th chord is a diminished triad with a double flat 7th (ff7), which is also considered a diminished 7th interval. It’s built 1-f3-f5-ff7. An Fsdim7 or Fs”7 is built using the notes Fs-A-C-Ef. That’s all minor 3rds! This is considered to be a fully diminished chord because of its consecutive minor 3rd intervals. This type of chord does not occur naturally in the major scale.
When it comes to using diminished chords as passing chords, usually full or diminished 7ths are used, and they’re not usually placed on the 7th scale degree. So diminished chords and their usage are not really associated with the diminished triad that occurs in the major scale on the 7th degree. Instead, you think of them as something different.
Musicians often use the term diminished on its own to refer to fully diminished 7th chords.
How to finger diminished chord shapes
One of the interesting features of fully diminished chords is that you can think of them as being built out of a series of minor 3rds. For example, a Bdim7 chord is built from the notes B-D-F-Af. Each chord tone is a minor 3rd, or three frets, above the previous one. If you were to continue another minor 3rd above the Af, you’d return to B.
To combine the notes of a Bdim7 to make chord shapes on the guitar you need to transpose some pitches up an octave. Here are the most common diminished chord fingerings.
You can see how it’s done in the Diminished 7th Chord Fingerings video.
How to play diminished 7th chord inversions on the guitar
Another interesting feature of diminished 7th chords is the way you play their inversions on the fretboard. Because diminished 7th chords are built out of all minor 3rds, you can simply move any fully diminished 7th chord fingering up or down three frets for an inversion (a reordering of the notes creating a new chord voicing). Move the same chord fingering three frets again, and you have the next inversion.
Continue in this way until you cycle back to your starting point an octave higher or lower, as seen here.