How to name and use 9, 11 and 13ths!
View the full lesson at Extension or Addition | JustinGuitar
I had a go at creating a basic overview. It’d be great if an advanced student could have a look at it.
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Some notes have been omitted from extended quadads to keep it simple: 11th chords have no 9th; 13th chords have no 9th or 11th. I’m aware this is not the whole picture.
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I haven’t added chord alterations for the same reason, except for the m7b5 chords where it’s part of the base quadad.
When constructing the chords, there are a few oddities I discovered. Are they correct?
1) Some suspended chords can have multiple names.
Csus2(add11) = Csus4(add9)
C9sus4 = C11sus2
I have seen Csus2sus4 on some occasions, but that seems a bit odd to me.
2) There are no ‘9sus2’ or ‘11sus4’ chords because they would duplicate notes.
‘C9sus2’ would have twice the note D, so it’s just a C7sus2.
‘C11sus4’ would have twice the note F, so it’s just a C7sus4.
3) We don’t speak of ‘diminished 6’ chords because we look at the 7th scale degree instead of the 6th.
The 6th = the bb7, so ‘Cdim6’ = Cdim7 or C full diminished.
Wow, you’ve been cogitating like a good 'un Jeff.
You’re using C rooted scale degrees and chords so:
E = 3 (major 3rd)
If the note E is present then the chord has definite quality - major - and will not be any type of suspended chord.
D = 2 = 9
F = 4 = 11
A = 6 = 13
Csus2(add11) =
Csus4(add9)
I have only kept the order derives from the scale degree numbering. But fundamentally there are four notes and these can be played in any order of pitch.
There is a Gsus2sus4 in the intro of this song: https://www.justinguitar.com/songs/sheryl-crow-if-it-makes-you-happy-chords-tabs-guitar-lesson-bs-426
As shown above 2 = 9 and 4 = 11.
But be careful with naming. 7, 9 and 11 chords are dominant (with a flat 7th) as opposed to maj7, add9, add11 chords.
C7sus2 and C7sus4 are dominant 7th chords with a Bb (flat 7th scale degree).
Yes.
There’s no light down that tunnel! 
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Yes, definitely not an exercise you want to do before your morning coffee.
Regarding sus2sus4 chords: on second thought, it would make more sense to say Csus2sus4 when there’s no 3rd. As opposed to ‘add9’ / ‘add11’ which implies the presence of the 3rd. For me at least, but maybe that’s just nitpicking.
(Headache coming in)
Csus2 = C-D-G
Csus4 = C-F-G
Csus2sus4 = C-D-F-G
C(add9) = C-D-E-G
C(add11) = C-E-F-G
C(add9)(add11) = C-D-E-F-G
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This sus2sus4 seems a bit like “minor major seventh”. The beauties of chord extensions and their names 
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This is what a sus chord is. sus is short for suspended meaning the 3rd(major or minor) is replaced with the 2nd or 4th interval of the scale. sus chord like power chord are neither Major or minor.
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Yes, I understand the theory and difference between suspending the 3rd and adding a note to the 3rd, hence why there are different chord names for sus and ‘add’ chords.
It just seems so that the chord naming isn’t always applied consistently depending on the consulted source/person. That’s where my mild ocd kicks in. 
Or two, or three!
It only makes sense without the 3rd. The presence of a 3rd automatically means it is the exact opposite of suspended but is in fact defined.
The internet is always right!