Natural classes

The major class features identify several categories of sounds which recur cross-linguistically in different phonological rules. Feature notation can also show why certain sounds behave similarly in similar contexts, within these larger classes. For instance, English /p/, /t/ and /k/ aspirate at the beginnings of words. All three may also be glottally reinforced at the ends of words. All three are unaspirated after /s/; and no other English phoneme has the same range of allophones, in the same environments. In feature terms, although /p/, /t/, /k/ differ in place of articulation, all three are obstruent consonants, and within this class, are [– voice, – nasal, – continuant]. A group of phonemes which show the same behavior in the same contexts, and which share the same features, constitute a natural class. More formally, a natural class of phonemes can be identified using a smaller number of features than any individual member of that class. As (12) shows, the class of voiceless plosives, /p/, /t/ and /k/, can be defined uniquely using only three features. If we subtract one of the plosives, we need more features, since we must then specify the place of articulation; and the same is true in defining a single plosive unambiguously.

Phonological rules very typically affect natural classes of phonemes. For example, medial voicing of /f/ to [v] in Old English, did not only affect that labial fricative, but also the other members of the voiceless fricative class, /s/ and /θ/. If we wrote a rule for /f/ alone, it would have to exclude the other voiceless fricatives, so that the input would have to include [+anterior, – coronal]; however, the more general fricative voicing rule in (13) requires fewer features to characterize the input, as we would expect when a natural class is involved.

This rule also neatly captures the connection between the process and its conditioning context, and therefore shows the motivation for the development: the fricatives, which are generally voiceless, becomes voiced between voiced sounds. This will often mean between vowels, as in heofon and hlaford; but it may also mean between a vowel and a voiced consonant, as in hæfde. If voicing takes place between voiced sounds, instead of having to switch off vocal fold vibration for a single segment and then switch it back on again, the vocal folds can continue vibrating through the whole sequence. Voicing the fricative in this context is therefore another example of assimilation, where one sound is influenced by another close to it in the utterance.