Phonemes and allophones

If two native English speakers were asked to say the sequence [bat] ten times, it is likely that none of their [b]s, [a]s or [t]s would sound exactly the same, but a third English speaker would nonetheless understand the same word, bat, every time. This is because speakers quickly learn to distinguish the differences that matter in their language from those that do not. An English-speaking child will very soon learn when a [p] sound, if voiced, may become a [b] and that this difference is important because pull and bull, pat and bat, path and bath and so on mean different things. And if the voicing distinction is important for [p]/[b], then it is likely also to be important for [k]/[g], [t]/[d] and so on.

The child will soon realize, however, that other differences are functionally unimportant in this sense. For most British English speakers, for example, the sounds represented orthographically by l at the end of cool and the beginning of leap are quite different, but speakers think of them as ‘the same’ sound. Many Cockney or Glaswegian speakers use a glottal stop [?] intervocalically in words like water or matter and yet the words will be perfectly well understood as if the speaker had produced a [t] (indeed, even RP speakers often use glottal stops in words like Gatwick or fortnight, where they generally pass unnoticed).

Speakers home in, then, on the distinctions (or oppositions as they are known) which are crucial for doing what speech sounds ultimately need to do in language, i.e. distinguish words, and ignore those that do not.