Which process involves turning each interval of a melody upside down?

Melodic inversion is the process of taking a melody and transforming each interval into its opposite. This means that if a melody moves up by a certain interval, in the inverted version it will move down by the same interval, and vice versa. This technique creates a new melody that maintains the same structure and relationships between notes but presents them in a different way.

For example, if the original melody ascends from C to E (a major third), the inverted melody would descend from C to A♭ (also a major third). This method is commonly used in musical composition to create variation while preserving the original thematic material.

The other options refer to different processes in music. Retrograde involves playing the melody backward, while retrograde inversion combines both inversion and retrograde, flipping the intervals and reversing the order. Serialism is a compositional method that uses a series of values to manipulate musical elements, such as pitch, rhythm, or dynamics, and does not specifically relate to the inversion of intervals in the manner described in the question.