A Dominant 9th chord adds a major 9th interval along with a minor 7th interval to the basic three notes of a major chord.
For example, a G9 chord consists of the notes G B D F and A (1-3-5-b7-9);
A C9 chord would be made up from the notes C F G Bb and D (1-3-5-b7-9).
The moveable Root 6 and Root 5 shapes are shown below:
A ninth chord can be used to great effect by sliding into it from a semi-tone up or down (e.g. 'Stormy Monday' as performed by The Allman Brothers).