For our final 9th chord we will be looking at the dominant family. Just as with the last two chords, we build the 7th chord first, then add the 9th. This is only the second dominant family chord we have made, so it's a very important chord that is the backbone of a lot of blues and funk playing!
In the case of a dominant 9th chord you simply take your major triad (1st, 3rd, 5th) add the b7th (to create the dominant 7th chord), and then the 9th. It is the addition of the 9th that changes the vibe of the chord to an even more relaxed sounding chord. Here is the formula applied to C major:
To put this into practice on the guitar, below is two common C9 chord shapes. Notice the notes you are playing and try to call each one out in theoretical terms (i.e. the 1st, 5th etc...).