In a C7 chord, the 7th is a B, but in the minor chord, the 7th is B flat. Why? Thank you!
They come from different keys. C7 is the dominant 5th from the key of F (one sharp), Cm7 is in 3 major keys. It is either a ii, iii, or vi chord (Bb, Ab, and Eb). In all three keys where Cm7 occurs B is flat.
Posted from my D2 with CM7GB