Ab Dominant 7

Ab Dominant 7, written Ab7, is built from Ab, C, Eb and Gb: the root, major 3rd, 5th and minor 7th.

Its 3rd and 7th, C and Gb, are its guide tones: they say what kind of chord it is, so voicings are built around them and the root is often left to the bass. D7, a tritone away, has the same two notes the other way up, so it can stand in for Ab7 as its tritone substitute.

In the progressions here, Ab7 is the V7 of the Major ii-V Progression in Db, the ♭VII7 of the Backdoor ii-V-I Progression in Bb, the ♭II7 of the Tritone Sub ii-V-I Progression in G, the ♭III7 of the Tritone Sub Turnaround in F, the II7 of the Cycle of Dominants in Gb and the I7 of the Whole-Step ii-V Chain in Ab.