In phonology, a natural class is a set of sounds in a language that share certain phonetic features. The sound system of every language includes several natural classes, each distinguished from other classes by certain features. A given natural class is described using the minimum number of features needed to include all sounds within the class and exclude all sounds outside the class. Some natural classes in a language are larger and include several other natural classes within them, while some may be small and include only one speech sound. Natural classes tend to behave in similar ways, participating in the same phonological rules.