Even though the Half Diminished chord is one of the seven related chords that emerge from the major scale — the one built on the 7th degree of the scale — it's rarely used in most modern music apart from jazzy tunes where it is quite common. Not one to get hung up on if you're just starting out.
The full Diminished chord has a very distinctive sound about it and it does come in very handy as a passing chord, one that slots in between two 'normal' chords. The fact that its scale, for once, is symmetrical (TsTsTsTs) means that if you move the chord shapes up or down 3 frets, you wind up with a new inversion of the same chord, and that makes it all a bit easier to keep track of.
Augmented chords have the same quirk. In this case, the underlying scale is also symmetrical (TTTTTT) so every 4 frets up or down brings you to a new inversion of the same chord. You'll find that moving between these inversions is a very effective way of stating the sound of augmented. They're also used more often than not as passing chords, especially when moving from a I to a IV chord.
6th chords come up rarely but have a nice sound about them. Even though it has an extra tone, it's not considered an 'extended' chord because the extra tone is just added to the triad. Extended chords always include the 7 and the added tones are 'above' the 7 or b7, in the next octave.
Once you understand that chords are just a bunch of tones that always can be related back to the 'pure' 1-3-5 of the major triad, it gets much easier to see them on the fretboard. Chords are sets of numbers and the trick is to be able to see the fretboard as numbers as well as note names. The names are fixed; the numbers move around according to the chord that's in play. There's plenty of time to get to the point where you can see the fretboard as an array of numbers, though, so don't feel like it's essential knowledge at this early stage.
There are just a couple more chord flavors to talk about before moving on to other topics. We'll first learn about the other two kinds of triads: Augmented and Diminished, and then we'll have a look at 6th chords. These are fairly uncommon flavors, so don't get too hung up on them at this stage, but like all chords, they can just be seen as altered major chords.
We learned long ago that major chords consist of the 1-3-5 of the major scale and that minor chords consist of the 1-b3-5 of the major scale (everything relates back to the major scale, even minor chords). Augmented chords consist of 1-3-#5 ... so the interval called the 'Perfect Fifth' has been 'augmented', or raised by one semitone. These chords have a very unstable sound because of that and are therefore not very common and are used as passing chords more than anything else, chords that lead from one stable chord to another.
The one interesting thing about them is that the intervals between the three notes are equal: four semitones (a major third is the proper term) between each note. What that means in practical terms is that there are in effect only 3 augmented chords, not 12. That's because augmented chords have inversions just as any chord, and because the constituent notes are four frets apart, the next inversion for each chord is four frets away. 12 divided by 4 equals 3. The video above lets you hear and see all of that in action. Below are the main shapes for Augmented chords
Move either of these shapes up or down 4 frets and you have a new inversion of the same chord. This means, in effect, that there are only three augmented chords!
We have already touched on Diminished Chords in the 'Chords defined' page. We learned that one of the diatonic chords (chords that arise naturally from the major scale) is a half-diminished chord, better know as 'minor seventh flat five', or m7thb5. That name already says 'diminished' because the Perfect Fifth has been flatted by one semitone. So the diminished triad is 1-b3-b5 ... both the third and the fifth have been flatted (or diminished) by a semitone. There is also another kind of diminished, one that has a 'double flat seven' note thrown in, which is a fancy way of saying '6'.
There's no need here to go into the reasonwhy it's called 'bb7' instead of 6, but this chord is sometimes called 'Diminished 7th', or 'Full Diminished' or even 'Fully Diminished'. The constituent notes of a Dim7 chord are 3 semitones apart, so it has the same quirk as augmented chords: there are, in effect, only 4 diminished chords, since the inversions of any given one are three frets apart. 12 divided by 3 equals 4. Below are the most common shapes for Diminished chords. Let's start with the 'Half Diminished', better known as the 'Minor seventh flat five" chord.
Half Diminished or 'm7thb5 chord shapes | 1 - b3 - b5 - b7
These are best played as four-note chords.
Full Diminished or Diminshed 7th chord shapes | 1 - b3 - b5 - bb7
These are also played as four-note chords. Notice that the flat seven has beed flatted, making it a double flat 7. Crazy, but true. The nice thing about full diminished chords is that you can move either of these two shapes up or down the fretboard by three frets and you will be playing the very same chord in a new inversion.
6th chords (Major) | 1 - 3 - 5 - 6
6th chords (minor) | 1 - b3 - 5 - 6