Carter's use of rhythm can best be understood with the concept of stratification. Each instrumental voice is typically assigned its own set of tempos. A structural polyrhythm, where a very slow polyrhythm is used as a formal device, is present in many of Carter's works. "Night Fantasies", for example, uses a 216:175 tempo relation that coincides at only two points over its 20+ minutes. This use of rhythm was part of his expansion of the notion of counterpoint to encompass simultaneous different characters, even entire movements, rather than just individual lines.