According to jazz theory, we can substitute a dominant seventh chord with another dominant seventh chord that shares the same tritone. The following image shows both chords. They both contain the tritone A-Eb/D#. The chords are also a tritone apart:

A short example where Db7 substitutes G7:

A descending half-step in a dominant chord is probably a tritone substitution.

The following image shows a cycle of fifths progression starting on E7 and ending on F7. The second system shows the same chords with tritone substitution applied to every other chord. This creates a series of chromatically descending dominant chords:

Some examples

Cb7 instead of F7:

Gb7 instead of C7:

The following excerpt is from Chopin's Prelude No. 8. It features a series of descending dominant sevenths, which is a variation of a cycle of fifths using tritone substitution (Bb7 = E7, Ab7 = D7, Gb7 = C7):

Chopin also uses tritone substitution in other preludes, such as Nos. 17 and 22.

The concept of tritone substitution is similar to that of augmented sixths in classical harmony. See Tritone Substitution and Augmented Sixth for more information.