In a language or dialect, a phoneme (from the Greek: φώνημα, phōnēma, "a sound uttered") is the smallest segmental unit of sound employed to form meaningful contrasts between utterances.
Thus a phoneme is a group of slightly different sounds which are all perceived to have the same function by speakers of the language or dialect in question. An example of a phoneme is the /k/ sound in the words kit and skill. (In transcription, phonemes are placed between slashes, as here.) Even though most native speakers don't notice this, in most dialects, the k sounds in each of these words are actually pronounced differently: they are different speech sounds, or phones (which, in transcription, are placed in square brackets). In our example, the /k/ in kit is aspirated, [kʰ], while the /k/ in skill is not, [k]. The reason why these different sounds are nonetheless considered to belong to the same phoneme in English is that if an English-speaker used one instead of the other, the meaning of the word would not change: using [kʰ] in skill might sound odd, but the word would still be recognized. By contrast, some other phonemes could be substituted (creating a minimal pair) which would cause a change in meaning: producing words like still (substituting /t/), spill (substituting /p/) and swill (substituting /w/). These other sounds (/t/, /p/ and /w/) are, in English, different phonemes. In some languages, however, [kʰ] and [k] are different phonemes, and are perceived as such by the speakers of those languages. Thus, in Icelandic, /kʰ/ is the first sound of kátur 'cheerful', while /k/ is the first sound of gátur 'riddles'.
In some languages, each letter in the spelling system represents one phoneme. However, in English spelling there is a poor match between spelling and phonemes. For example, the two letters sh represent the single phoneme /ʃ/, while the letters k and c can both represent the phoneme /k/ (as in kit and cat).
Phones that belong to the same phoneme, such as [t] and [tʰ] for English /t/, are called allophones. A common test to determine whether two phones are allophones or separate phonemes relies on finding minimal pairs: words that differ by only the phones in question. For example, the words tip and dip illustrate that [t] and [d] are separate phonemes, /t/ and /d/, in English, whereas the lack of such a contrast in Korean (/tʰata/ is pronounced [tʰada], for example) indicates that in this language they are allophones of a phoneme /t/.
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