Bb Add9 (Bbadd9)
Also written as A# Add9 (A# and Bb are the same key).
Notes in Bb Add9
Inversions
Same notes as Bb Add9, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
A# in the bass
1st inversion
D in the bass (Bbadd9/D)
2nd inversion
E# in the bass (Bbadd9/E#)
3rd inversion
B# in the bass (Bbadd9/B#)
Open voicings
A close-position chord like Bb Add9 can sound cluttered in the middle of the keyboard. Jazz players open it up by dropping one note down an octave, spreading the same notes wider without changing the chord.
Drop 2
E# on the bottom
Drop 3
D on the bottom
Bb Add9 is built from A#, D, E# and B#.
Take a plain major triad and drop a ninth on top without adding a seventh underneath it, and the result is bright, open and thoroughly modern. Unlike a full ninth chord it never tips into jazzy territory, which is exactly why it's a staple of pop and singer-songwriter piano and guitar parts.