The 12 bar blues progression, and variations on its theme, is the basis for all blues, some jazz, and many rock & roll songs. It uses all dominate 7th chords.
Ab Major has 4 flats: Bb, Eb, Ab, and Db. The relative minor of Ab Major is F minor. Ab Major is tonally identical to and can be referred to as G# Major.
C major uses no sharps or flats. The chords in this key are: C maj, D min, E min, F maj, G maj, A min, B diminished. The relative minor of C Major is A minor.
If you start at middle C and work your way up the musical scale by intervals of perfect fifths you will eventually play all twelve notes, completing the circle.
Cold Modulation refers to the practice of changing keys in a musical piece by applying an immediate key change with no preparatory harmonic progression.