Originally Posted By: ZeroZero
The A7 in the key of C in a I,VI,II,V,I converts the Am (6th chord) into a V chord of the II minor.
This briefly brings the chords out of the key signature, but because the IIm then goes to a V and a I of C we rapidly return.


What you say is absolutely correct and a good analysis of the A7b9 function in the key of C. But a simpler way to approach a flat 9 chord, regardless of the key is to look at it as an alt chord. That makes available all the neat tensions (b5, #5, b9, #9, etc.).