Webster tells you all you need to know, but the difference between a 9th and an add9 chord should probably be clarified.

Simply, where ninth chords such as F9 require the inclusion of the dominant seventh, chords such as F add9 should not have the seventh.

Hence, the notes in F add9 are F A C G, and the notes in Fm add9 are F Ab C G.

Ninth (?), a. [From Nine; cf. AS. nigo&edh;a.]


Following the eight and preceding the tenth; coming after eight others.


Constituting or being one of nine equal parts into which anything is divided.


© Webster 1913.

Ninth, n.


The quotient of one divided by nine; one of nine equal parts of a thing; the next after the eighth.

2. Mus. (a)

An interval containing an octave and a second.


A chord of the dominant seventh with the ninth added.


© Webster 1913.

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