The week I learned the difference between dominant 9th, major 9th, and add9 chords. For example, a C9 chord is a C dominant chord (C, E, G, Bb) with an added 9th (C, E, G, Bb, D). A C major 9th chord is a C major 7th chord (C, E, G, B) with an added 9th (C, E, G, B, D). A Cadd9 chord is a C chord (C, E, G) with an added 9th (C, E, G, D). The Wikipedia page for 9th chords is very nice.