An added ninth chord (also called a sus2 chord, see next paragraph) is composed using a root/1st, major 3rd, perfect 5th and major 9th (or R/1-3-5-9) intervals played simultaneously. Generally written as Cadd9 or C(add9), it is good practice to simply use an uppercase C, followed by a super scripted add9 to represent it in writing (ie: Cadd9).
In order for a chord to be called an add9 chord it must not contain a seventh degree whether it be major, minor or diminished. This lessens the overall "major" feel of the chord and also opens up some melodic possibilities that would otherwise be unavailable. Some texts refer to the add9 chord as being equivalent to a sus2 chord however this is incorrect, the sus2 chord infers there is no third degree present in the voicing, where as the add9 chord does have a third present and it is presumed that the added ninth interval should come from the second octave above the root, however in practice this is not always the case.
Added ninth chord profile
|Intervals||root/1st, major 3rd, perfect 5th, major 9th or R/1-3-5-9|
|Common names (examples in C)||Cadd9, C(add9), Csus2, Cadd2|
Added ninth chord voicings
Added ninth chord audio sample(s)
|Added ninth chord voicing 1|