[Dave Wallbridge]

I'm working through the most recent lessons and am puzzel'd. In the key of F, why does a B flat creep in, where in all other key's sharps are used to cater for 1/2 steps and are used in the key signature. Why not call this an A sharp?

I know there's gonna be a reason but I've looked and looked and perhaps the obvious isn't staring me in the face.

Thanks in advance for the answer.

Best regards Dave W