WHAT KEY IS THE PIECE IN? And is the chord written as B b5 (that is, a B with a flat 5th) or is it written as Bb 5 (that is, a B flat open 5 chord -no 3rd). THE KEY WILL MAKE THAT USUALLY OBVIOUS.

I am sorry to disagree but the key may not tell you want you want. Look at The Christmas Song, there are many chords which do not relate to the key the music was written.

It is the chord progression and like Diki has said, the notes in that measure that help you understand the chord. How does it relate to the song, the key, the written notes and the chord progression.

Sorry to disagree Diki, John C.