Bb Major 7th (Bbmaj7)
Also written as A# Major 7th (A# and Bb are the same key).
Notes in Bb Major 7th
Inversions
Same notes as Bb Major 7th, 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 (Bbmaj7/D)
2nd inversion
E# in the bass (Bbmaj7/E#)
3rd inversion
A in the bass (Bbmaj7/A)
Open voicings
A close-position chord like Bb Major 7th 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
The notes A#, D, E# and A make up Bb Major 7th.
Top a major triad with a major seventh instead of a minor one and the chord stays inside its own key rather than pulling anywhere. The result is warm and reflective, a little more sophisticated than a plain triad, and it shows up constantly in soul, bossa nova and any film cue that wants a major chord to sound thoughtful rather than triumphant.