An allophone is an imperceptibly slight variation on a given vocal sound of a language. For example, the letter k in kill and skill may sound the same to most people, but are very different sounds under critical phonetic analysis. If the two variations of the letter were mispronounced, by say a non-native speaker, the meaning of the word will not have changed, but most people will instantly hear it initially as less than intelligible.
The Greek root phone means “sound,” and the prefix allos means “other.” In linguistics, the study of language, a phoneme is the smallest distinctive unit of sound. The consonant k is a phoneme, and replacing it with another sound unit such as t will change the meaning of the word. Each of these singular phonemes however can have multiple ways to be voiced.
“Kill” is aspirated — its pronunciation is accompanied by an explosive puff of air. With a palm held in front of the mouth, “skill” is demonstrably pronounced unaspirated. In phonology, the study of how humans create the sounds of language, phonemes are characterized by different air flow and contrasting positions of the lips, tongue and other parts of the vocal tract. For example, vowels and consonants like m with minimal manipulation of air flow are called sonorants.
Other vocal articulations include the characteristic “hiss” of sibilants like z, and nasals such as the consonant m in “mouse” when air flow is redirected through the nose. The latter example is an allophone of the m in “manipulate.” Phonemes in a given language may have any number of allophones. The consonant t in English has six of them, and native speakers are hardly aware of any difference in them during normal conversation. In Mandarin Chinese however, the aspirated and unaspirated t are completely separate phonemes whose respective use changes the meaning of the word.
An allophone can be an interchangeable free variation. This is what most commonly differentiates dialects and accents, such as British English versus American English. One may find the other nearly unintelligible upon first encounter, but since the meaning of words do not change with different pronunciation, comprehension is usually quickly established.
Most allophones in a given language or dialect are not interchangeable and are said to occur in complementary distribution. A particular allophone must occur in a particular phonetic context, while a different one can be expected in another context. In standard English, the k in “kill” is always aspirated when it occurs at the beginning of a word. Such definitions of the context in which a particular allophone is expected to be vocalized are collectively called allophonic rules.