What describes the harmony based on fourths rather than thirds?

Quartal harmony specifically refers to chords built on intervals of fourths instead of the traditional thirds commonly used in classical harmony. This approach to harmony can create a more modern and ambiguous sound, often associated with jazz and 20th-century music. The use of fourths allows for distinctive voicings and textures that differ significantly from the triadic structure found in tonal music.

In quartal harmony, the chords are stacked in intervals of a perfect fourth, leading to a sound that is often perceived as less conventional or "open" compared to traditional tonality rooted in thirds. This method can also facilitate greater harmonic exploration and flexibility, allowing composers and musicians to deviate from established tonal norms.

The other options provided do not fit the description of harmony based on fourths. Melodic inversion refers to altering the direction of a melody while retaining the original intervals. Bitonality and polytonality involve the simultaneous use of two different keys or tonal centers, but neither specifically defines a harmony based on fourths. Thus, identifying quartal harmony is essential in understanding modern compositional techniques and harmonic structures.