We generally are not aware of the phonetic properties or features that distinguish the phonemes of our language. Phonetics provides the means to describe these sounds, showing how they differ; phonology tells us which sounds function as phonemes to contrast the meanings of words.
In order for two phonetic forms to differ and to contrast meanings, there must be some phonetic difference between the substituted sounds. The minimal pairs seal [sil] and zeal [zil] show that [s] and [z] represent two contrasting phonemes in English. They cannot be allophones of one phoneme since one cannot replace the [s] with the [z] without changing the meaning of the word. Furthermore they are not in complementary distribution: both occur word initially before the vowel [i]. They therefore are phones which function as allophones of the phonemes /s/ and /z/. From the discussion of phonetics in chapter 6, we know that the only difference between [s] and [z] is a voicing difference; [s] is voiceless and [z] is voiced. It is this phonetic feature that distinguishes the two words. Voicing thus plays a special role in English (and in many other languages). It also distinguishes feel and veal [f] / [v] and cap and cab [p] / [b]. When a feature distinguishes one phoneme from another it is a distinctive feature (or a phonemic feature). When two words are exactly alike phonetically except for one feature, the phonetic difference is distinctive, since this difference alone accounts for the contrast or difference in meaning.
