Bitonality is the practice of using two different keys simultaneously. It was first introduced during the twentieth century by classical composers. For instance, a musical composition may include C Major and F# Major to be played concurrently, at the same time to provide a unique harmonic function. Today, it’s most commonly found in jazz music, which is why some songs – particularly instrumentals – sound so abstract, as if the musicians are playing random notes unconnected to one another.
Bitonality is the most common type of polytonality or polyharmony and was originally popularized in Stravinsky’s ballet Petrushka, which creates a dramatic effect and is also used to create tension, which is generally later resolved with a more harmonic element.