In phonology, a natural class is a set of phonemes in a language that share certain distinctive features.
[1] A natural class is determined by participation in shared phonological processes, described using the minimum number of features necessary for descriptive adequacy.
To give a further example, the system of Chomsky and Halle defines the class of voiceless stops by the specification of two binary features: [-continuant] and [-voice].
This means that all sounds with either the feature [+continuant] (able to be lengthened in pronunciation) or [+voice] (pronounced with vibration of the vocal cords) are excluded from the class.
These are not relevant to the description of the class and are unnecessary, since the features [-continuant] and [-voice] already include all voiceless stops and exclude all other sounds.