ormation; the rules for the possessive forms of nouns and for the third person singular of the present tense of verbs are similar in this respect. The rules are more explanatory, however, if they show that these phonemes behave in a similar way because they form a natural class, or set, whose members are defined by a common property. In the case of these plural forms, the phonemes are all, and only, those that have a high-frequency fricative component; they may be called sibilant, or strident, phonemes.
Phonemes catalogued in the lexicon may be also classified in terms of features. To be descriptively adequate from a linguistic point of view, the set of features must be able to provide a different representation for each of the words that is phonologically distinct; and if the feature is to have an explanatory power it must also be able to classify phonemes into appropriate natural classes as required by the phonological rules.
As a result of studying the phonemic contrasts within a number of languages, Roman Jakobson, Gunnar Fant, and Morris Halle concluded in 1951 that segmental phonemes could be characterized in terms of twelve distinctive features, all of which were binary. Norman Chomsky and Mor
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