To play the A#min7b5 chord, place the index finger on the 1st fret of the top g-string, middle finger on the 1st fret of the C-string, and ring finger on the 1st fret of the bottom A-string. Let the other string ring open.
The Bbmin7(b5) or A#min7(b5) chord is a minor-seventh-flat-five chord, also known as a half-diminished-seventh chord, and contains the notes Bb-Db-Fb-Ab. That Fb is not a typo and is called an "F flat", which is the enharmonic equivalent to an E note.
To build a Bbmin7(b5), take the minor seventh chord formula (1-b3-5-b7) and apply it to the Bb major scale (Bb, C, D, Eb, F, G, A). Flat the fifth scale degree to make the minor-seventh-flat-five formula 1-b3-b5-b7.
Identify the first, flat third, flat fifth, and flat seventh scale degrees in the Bb major scale which are Bb-Db-Fb-Ab to form the Bbmin7(b5) chord.
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