What is the combination of retrograde and melodic inversion called?

The combination of retrograde and melodic inversion is known as retrograde inversion. This term refers to a musical process where a melody is first inverted—meaning the intervals between the notes are flipped—and then played in reverse order. This technique is significant in various forms of composition, particularly in serialism and 20th-century music, allowing for the exploration of thematic transformation and variation.

The other options represent different musical concepts. Polytonality involves the simultaneous use of two or more different tonalities, creating a complex harmonic texture. Quartal harmony utilizes chords built on intervals of fourths rather than the traditional thirds, producing a unique sound characteristic of modern music. The twelve-tone row is a specific compositional technique developed by Arnold Schoenberg in which composers arrange the twelve pitches of the chromatic scale in a specific sequence to provide the basis for a piece of music. Each of these concepts serves its purpose in music theory and composition, but they do not pertain to the process of combining retrograde and melodic inversion.