If you want to use a scale, use Mixolydian.
But I prefer referencing a chord, a dominant thirteenth chord (from a major scale.)
The jazz chord name system doesn't reference any key or scale (the chords are never in major or Mixolydian, or whatever) so I think it's more logical to reference a default chord rather than scale.
So the reference chord, the dominant thirteenth, rooted on
G, from a hypothetical
C major scale, will contain tones:
G B D F A C E G.
Use those tones and their intervals above the root, in simple and compound form, to get the default chord extensions and modifications.
Simple intervals are an octave or smaller, compound intervals are a simple interval plus an octave. Example, a sixth is simple (smaller than an octave), add an octave to a sixth to get the compound interval of a thirteenth. Simple and compound pairs: 2nd & 9th, 4th & 11th, 6th and 13th.
The seventh and larger intervals (9th, 11th, & 13th) are called extensions and just get added as a number. Any extension use implies the presence of all extensions down to the seventh. So a
13 chord implies the presence of the 7th, 9th, and 11th in the chord even though by convention some of those intervals are often omitted. (Usually the seventh is obligatory with higher extensions.)
When a simple interval is used, use
add6. The label
sus implies the sus tone is used instead of the chord's third. The label
add means add the tone to a basic triad. So
sus4 normally means the third isn't played, example
G C D. A
Gadd6 is adding a sixth to the
G triad, example
G B D E.
Raise or lower any of the added numeric figures with a sharp to raise or flat to lower, except for the seventh which is raised with either
△ to mean a major seventh or
dim7 for a diminished seventh.
...someone mentioned "add" chord only works when the additional note is higher then 7th. Are there no add2 chords then?
I think that is trying to say
Gadd2 should really be written
Gadd9, because it's more conventional to see a
2 as a
Gsus2, and because in practice people are not very picky about into which which octave the
add2/add9 gets placed.
If you did allow both
add9 to be used, then
add2 should be a whole step above the root and the
add9 should be a major ninth above the root. In both cases the third of the chord should be present. In the case of
sus2 the third should not be present as it is "suspended" in the tone of the
...Is that why C6 for example is so strange looking? (i'd expect it to be a dominant chord with added 6th...
If you follow the system, I think you will see that given tones
C E G A Bb, the seventh implies a chord extended with compound intervals in which case the
A is considered a thirteenth regardless of whether the
A is actually a sixth or thirteenth above the root. It's theoretically a thirteenth.
Be mindful of potentially confusing figures, enharmonic naming, and more conventional labels. Example,
G7b5 lowers the fifth of the chord from
Db ...that means there is no
D natural in the chord. Writing
G7#4 would add a
C# which is enharmonically equal to
Db, but because that chord name doesn't alter the fifth it would mean
G B C# D F where the
D natural is included. The convention is to use
G7b5 and the tones would be
G B Db F with no
The system is a bit confusing, but once you get a handle on it, it packs a lot of chord information into a concise symbol.