A Substitute for a Dominant 7th is the Tri-Tone substitution. Ex. Key of C. When playing a G7 you could substitute a Db7. The reason why this works is that the two chords share the same guide tones (the 3rd and the 7th of a chord). The major 3rd of G7 is the minor 7th of Db7 and the minor 7th of G7 is the major 3rd of Db7.