Watch the A's In The Bass online guitar lesson by Brad Carlton from Chord Studies: Double-Stops Vol. 1
This example demonstrates coming down in thirds in the A Mixolydian mode. We start with the root and the third on the 14th fret and then move down a whole step to get our A9. This can also be thought of as a G/A, but since we have an A in the bass and the C♯ stilling ringing in our ears from the previous chord it ends up sounding like an A9, even though there is no third in the chord! The last two shapes are familiar patterns, so why not try linking this to previous examples?