What harmony involves the simultaneous use of two or more keys?

The concept of harmony that involves the simultaneous use of two or more keys is termed polytonality. This technique is characterized by the layering of different tonal centers, creating a complex and rich sound texture. Composers who use polytonality may employ two or more keys at once, allowing for a fascinating interplay between the different harmonic structures. This approach can evoke varied emotional responses and contrast between the distinct tonalities, leading to innovative musical expressions.

In contrast, other terms describe different harmonic concepts. Bitonality refers specifically to the use of two keys simultaneously, making it more of a subset of polytonality. Atonality lacks a clear tonal center or key altogether, which is a different approach entirely. Quartal harmony is built on the stacking of fourths rather than the use of multiple keys. Thus, polytonality is the most comprehensive term appropriate for the question presented.