What does polytonality mean?

Polytonality refers to the simultaneous use of two or more keys within a piece of music. This technique involves layering musical lines or chords based on different tonal centers, resulting in a complex and rich soundscape. Composers who employ polytonality often create tension and unique harmonic textures by combining disparate keys, leading to innovative musical experiences.

The other options describe different concepts in music theory that do not encompass the definition of polytonality. A single key relates to monophonic or simple harmonic structures, harmonizing melodies often involves the use of consistent chords that support a melody without shifting into multiple keys, and modal movement refers to changing tonalities but does not imply the simultaneous presence of different keys. Thus, the concept of polytonality is specifically characterized by the coexistence of distinct tonal centers.