Cole Porter (1891–1964), “What Is This Thing Called Love?” from Wake Up and Dream (1929)

Cole Porter is one of the most celebrated American Jazz musicians. He claimed that the alternations of major and minor key centers in this song were inspired by Moroccan folk music. The inclusion of alternating major and minor here, however, is made possible by the similarities between the German-sixth chord and the Tritone Substitution of V7/V.

Although these two chords are equivalent in terms of structure, they differ in their voice-leading conventions and to some extent in their function: while the German-sixth chord appears most commonly as a dominant preparation, the Tritone Substitution of V7/V is (as its name implies) a dominant substitute.