I was messing around tonight and stumbled upon this 4 chord progression that cycles nicely. I played these four chords only using three notes max: F = F, A, C B7 = B, A, D# Bb7 = Bb, D, A Eb7 = Eb, G, Db The progression has a great chromatic tritone decent thats makes its real fun to solo over. I was able to use a mix of F major pentatonic and added chord tones with a lot of success. I don't understand what key the progression is in. Will someone help enlighten me?