In phonetics, an allophone is one of several similar speech sounds (phones) that belong to the same phoneme. A phoneme is an abstract unit of speech sound that can distinguish words: That is, changing a phoneme in a word can produce another word. Speakers of a particular language perceive a phoneme as a distinctive sound in that language. An allophone is not distinctive, but rather a variant of a phoneme; changing the allophone won't change the meaning of a word, but the result may sound non-native, or be unintelligible. (There is debate over how real, and how universal, phonemes really are. See phoneme for details.)
Every time a speech sound is produced, it will be slightly different from other utterances. Only some of the variation is significant (i.e., detectable or perceivable) to speakers. There may be complementary allophones which are distributed regularly within speech according to phonetic environment, as well as notable free variants, which are a matter of personal habit or preference. Not all phonemes have significantly different allophones.
In the case of complementary allophones, each allophone is used in a specific phonetic context and may be involved in a phonological process.
Besides, there are many different allophonic processes in English, like lack of plosion, nasal plosion, partial devoicing of sonorants, complete devoicing of sonorants, partial devoicing of obstruents, lengthening and shortening vowels, dentalisation and retraction.
Because the choice of allophone is seldom under conscious control, people may not realize they exist. English-speaking people may become aware of the difference between two allophones of the phoneme /t/, namely unreleased [t̚] and aspirated [tʰ], if they contrast the pronunciations of the following words:
If a flame is held before the lips while these words are spoken, you may notice that it flickers more during aspirated nitrate than during unaspirated night rate. The difference can also be felt by holding the hand in front of the lips. For a Mandarin speaker, to whom /t/ and /tʰ/ are separate phonemes, the English distinction is much more obvious than it is to the English speaker who has learned since childhood to ignore it.
Allophones of English /l/ may be noticed if the 'light' [l] of leaf [ˈliːf] is contrasted with the 'dark' [ɫ] of feel [ˈfiːɫ]. Again, this difference is much more obvious to a Turkish speaker, for whom /l/ and /ɫ/ are separate phonemes, than to an English speaker, for whom they are allophones of a single phoneme.
However, there may be several such allophones, or the linguist may prefer greater precision than this allows. In such cases a common convention is to use the "elsewhere condition" to decide which allophone will stand for the phoneme. The "elsewhere" allophone is the one that remains once the conditions for the others are described by phonological rules. For example, English has both oral and nasal allophones of its vowels. The pattern is that vowels are nasal only when preceding a nasal consonant within the same syllable; elsewhere they're oral. Therefore, by the "elsewhere" convention, the oral allophones are considered basic; nasal vowels in English are considered to be allophones of oral phonemes.
In other cases, an allophone may be chosen to represent its phoneme because it is more common in the world's languages than the other allophones, because it reflects the historical origin of the phoneme, or because it gives a more balanced look to a chart of the phonemic inventory. In rare cases a linguist may represent phonemes with abstract symbols, such as dingbats, so as not to privilege any one allophone.