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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.
Classes are called by their manners of articulation, their places of articulation, voicing, and continuance as determined by the International Phonetic Alphabet. For example, the set containing the sounds /p/, /t/, and /k/ is a natural class of voiceless stops in American Standard English. This class is one of several other classes, including the voiced stops (/b/, /d/, and /g/), voiceless fricatives (/f/, /θ/, /s/, /ʃ/, and /h/), sonorants, and vowels.
The class of voiceless stops is described by two features: [-continuant] and [-voice]. This means that any sound with both the feature [-continuant] (not able to be pronounced continuously) and the feature [-voice] (not pronounced with vibration of the vocal chords) is included in the class. This correctly describes all voiceless stops and does not describe any sounds besides voiceless stops.
By implication, the class is also described as not having the features [+continuant] or [+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 chords) are excluded from the class. This excludes all natural classes of sounds besides voiceless stops. For instance, it excludes voiceless fricatives, which have the feature [+continuant], voiced stops, which have the feature [+voice], and liquids and vowels, which have the features [+continuant] and [+voice].
Voiceless stops also have other features, such as [+consonantal] and [-lateral], that are not added 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.
In addition, members of a natural class will behave similarly in the same phonetic environment, and will have a similar effect on sounds that occur in their environment.
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