This inversion uses a barre across the guitar's D, G and B strings with your first finger.
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This inversion is moveable and has it's root on the bottom E string of the guitar. For example moving the chord down one fret would create an A add9 chord and moving up two frets would create a C add9 guitar chord.
Bb add9 uses these notes Bb, D, F, C
If we look at the notes of the Bb major scale:
Bb, C, D, Eb, F, G, A, Bb, C
We can see that the Add9 chord has used the 1, 3, 5, 9 from the scale, in our case Bb, D, F and C
This chord is used in lots of songs in the key of Bb, F and Eb.
You'll find lots more ways to play different add9 guitar chords here.