Add9 (Stable?)

i'm referring to a major chord: C-E-G with an added ninth D.

is this considered a stable chord?

the melody usually is on that ninth (D), but ..how does it get away with it? that should be tension.

i've heard this taylor swift - style which has the chorus on the added ninth. normally that would be a major second (to the harmony), which is an unusual tone to start a *concluding melody* like a chorus, but it would be somewhat acceptable if it would resolve to the root. but here it "resolves" to the fifth, like a half cadence. finally it rests on the second/ninth as a conclusion (the second/ninth sounds more stable than the fifth)

so what's with this added ninth, not just as chord embellishment, but as the central tone in the melody line?

If you check Eric Whitacre, all his compositions have added tones. In contemporary music, added notes to simple triads are no longer a tension to be resolved. Debussy was already using such chords, even in his simple compositions (his famous Cakewalk for example).

Another way to look at extended notes in the scale is to first note whether there is a seven in the chord. If it's a flat seven and the third is major, like E in your example, then the chord is dominant and wants to be resolved, unstable in your parlance. But if the third is minor, Eb, then the added ninth or second is color, melody or not. If the third is major but there is no seven, you have to go by context. In Taylor Swift's style, the ninth is color, or just a suspension that could resolve to the root but doesn't. That's acceptable. 

aha, thanks guys.

usually when i see a melody line in A minor i assume the chords with which is accompanied are also in A minor, or they could be in F major (because F is the root between F-A, so the harmony has it's root in the base). or they could be in D minor because D-A has D as the root also.

i get intrigued when the melody is orchestrated with chords that don't have the base (or center) of the harmony in them. like if i would put the E minor chord/harmony underneath the A minor melody. i guess as a whole the A would be viewed as an added forth to the harmony of Eminor. i usually think of the melody as being the important part, and the chords just an orchestration (which can be done in all sorts of ways). so i view it like the A is orchestrated with the chord on it's fifth (the E).

and this is how i view the add9 situation also. the melody not being the ninth of something, but the root of itself and is accompanied with a chord/harmony on it's (minor) seventh (C major is the seventh degree of the D minor key/harmony).

this is the view from the melody perspective. i guess i have to further investigate, because what intrigued me the most in this song is that the melody of the verses and prechoruses are in the harmony of C (i think, but this is where i could be wrong). and the chorus comes in the harmony of D minor. (again from the melody/cantus firmus perspective). this modulation from C to D is what's unusual. when i was transcribing the melody i double and triplet checked because it sounded wrong, or unstable, especially for a chorus.