In jazz music a tritone substitution is the use in a chord progression of a dominant seventh chord (major/minor seventh chord) that is three steps (a tritone) away from the original dominant seventh chord.

The reason a chord a tritone away may be substituted is because dominant seventh chords contain a tritone between their third and seventh members (B and F in a dominant seventh on G: G-B-D-F) and this tritone is shared with the dominant seventh chord whose root is a half octave away (from G, a dominant seventh on C#: C#-E#-G#-B or, enharmonically, Db: Db-F-A-Cb, E# = F and Cb = B).

Classical theory would notate the "substitute" as an augmented sixth chord, specifically the enharmonically equivalent German sixth which serves as a substitute for the dominant of the dominant (V/V) (Satyendra 2005, p.55).

Below is the original dominant-tonic progression, that progression with the tritone substitution, and the same progression with the substitution notated as an augmented sixth chord:

Tritone substitutions are a common technique in jazz and were first used by musicians such as Duke Ellington, Art Tatum, Coleman Hawkins, Roy Eldridge and Benny Goodman. Tritone substitutions are a defining part of Coltrane changes.

Source

• Stein, Deborah (2005). Engaging Music: Essays in Music Analysis. New York: Oxford University Press. ISBN 0195170105.
  • Satyendra, Ramon. "Analyzing the Unity within Contrast: Chick Corea's Starlight".