To play the A#7 chord, perform a barre by pressing your index finger on the top g-string, E-string, and bottom-A string on the 1st fret and middle finger on the 2nd fret of the C-string.
The Bb7 chord (also known as an A#7 chord) contains the notes Bb-D-F-Ab.
To build a Bb7 chord, take the dominant seventh formula (1-3-5-b7) and apply it to the B major scale (Bb, C, D, Eb, F, G, A).
Identify the first, third, fifth, and flat seventh scale degrees in the Bb major scale which are Bb-D-F-Ab.
Notice how the seventh scale degree found in the chord is lowered a half step as indicated by the "flat" or "b" that precedes the number "b7". In this case, the seventh scale degree of a Bb major scale – a "A" note is lowered a half step to a "Ab" note to make the chord a dominant seventh chord.
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