My guess to this is that you call it a 9th, 11th, or 13th, if that note(the 9th, 11th, or 13th, is the top voice in the chord. It would be called sus 2, sus 4 or 6th if it was not the top voice. Is that basically how it goes? I'm trying to get more into chord soloing and this would help clear things up a bit.
Generally if a 9th or 11th is substituted for the 3rd it is called a sus2 and sus4 respectively. If however the 3rd is present and the 9th or 11th is added then it is called an add9 or add11 chord. I don't think it has to be the top voice necessarily, it's just how it goes. I've also heard that it depends on where the melody is headed. When voiceleading for example, a #5 is called a b13 when it is moving downwards from a higher note. This is by no means a rule, I've just heard someone say it's a bit tidier.
Technically speaking, if the extension is a 9th away from the root, it is an add9. If it is a 2nd away from the root, it is an add2. Usually people call these by their higher octave names either way, so you can get away with saying Dadd9 or F#9 rather than getting more specific.

As far as the #5/b13 goes, changing the fifth will alter the function of the chord, i.e. making it augmented, whereas a b13 is just an extension. It's about context.
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^ irrelevant. there is no such thing as add2. it's named purely on function, so it's going to be an add9, even if the chord is voiced C D E G (from bass to soprano, within an octave).

octaves have nothing to do with it. it's all based on the function of the note. it has nothing to do with the soprano voice.

it's a sus2 or a sus4 when the 2 or the 4 REPLACES the 3. e.g. Cmaj = C E G; Csus2 = C D G; Csus4 = C F G. traditionally there's no such thing as a sus2 -- that's called a retardation, since the D is held back and rises to the E. the sus4 is the only true suspension, since the F is suspended and then falls to the E. but you don't really need to know that here. otherwise they become 9 and 11.

a 6th is involved if you have a triad with an interval a sixth above the root. if a chord with the function of a C chord has the sixth (making C E G A), it's a C6. be careful -- if the chord functions as an A -- it'd be an Am7.

9th, 11th, and 13th also play a major role when the 7th of a chord is already present and further extensions are added. C E G F would be Cadd11 (weird chord, btw), but C E G Bb (D) F would be C11, regardless of the presence of the D.

those are the general guidelines, but correct chord identification requires a solid knowledge of harmonic function. if you don't have that, you're probably going to be shit out of luck. if these are things you don't know, go back and solidify your theory more.

Quote by Sóknardalr
When voiceleading for example, a #5 is called a b13 when it is moving downwards from a higher note.

it'd be called a b6 here, not a b13. we're talking about melody here, which isn't so much under the realm of harmonic function as it is the rules of music notation. it's not about chords, but melody.
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Last edited by AeolianWolf at Jun 23, 2011,