Allophone

In phonology, an allophone is one of multiple possible spoken sounds, or, used to pronounce a single phoneme in a particular language. For example, in English, the voiceless plosive and the aspirated form are allophones for the phoneme, while these two are considered to be different phonemes in some languages such as Central Thai. Similarly, in Spanish, and are allophones for the phoneme, while these two are considered to be different phonemes in English.
The specific allophone selected in a given situation is often predictable from the phonetic context, with such allophones being called positional variants, but some allophones occur in free variation. Replacing a sound by another allophone of the same phoneme usually does not change the meaning of a word, but the result may sound non-native or even unintelligible.
Native speakers of a given language perceive one phoneme in the language as a single distinctive sound and are "both unaware of and even shocked by" the allophone variations that are used to pronounce single phonemes.

History of concept

The term "allophone" was coined by Benjamin Lee Whorf circa 1929. In doing so, he is thought to have placed a cornerstone in consolidating early phoneme theory. The term was popularized by George L. Trager and Bernard Bloch in a 1941 paper on English phonology and went on to become part of standard usage within the American structuralist tradition.

Complementary and free-variant allophones

Each time a speaker vocalizes a phoneme, they pronounce it differently from previous iterations. There is debate regarding how real and universal phonemes are. Only some of the variation is perceptible to listeners speakers.
There are two types of allophones: complementary allophones and free-variant allophones.
Complementary allophones are not interchangeable. If context requires a speaker to use a specific allophone for a given phoneme, the possible allophones are said to be complementary. Each allophone from a complementary set is used in a specific phonetic context and may be involved in a phonological process.
Otherwise, allophones are free-variant; speakers choose an allophone by habit or preference.

Allotone

An allotone is a tonic allophone, such as the neutral tone in Standard Mandarin.

Examples

English

There are many allophonic processes in English: lack of plosion, nasal plosion, partial devoicing of sonorants, complete devoicing of sonorants, partial devoicing of obstruents, lengthening and shortening vowels, and retraction.

Aspiration: In English, a voiceless plosive is aspirated if it is at the beginning of the first or a stressed syllable in a word. For example, as in pin and as in spin are allophones for the phoneme because they cannot be used to distinguish words. English-speakers treat them as the same sound, but they are different: the first is aspirated and the second is unaspirated. Many languages treat the two phones differently.
Nasal plosion: In English, a plosive has nasal plosion if it is followed by a nasal, whether within a word or across a word boundary.
Partial devoicing of sonorants: In English, sonorants are partially devoiced after a voiceless sound in the same syllable.
Complete devoicing of sonorants: In English, a sonorant is completely devoiced after an aspirated plosive.
Partial devoicing of obstruents: In English, a voiced obstruent is partially devoiced next to a pause or next to a voiceless sound within a word or across a word boundary.
Retraction: In English, are retracted before.

Since the choice among allophones is rarely under conscious control, few people realize their existence. English-speakers may be unaware of differences between a number of allophones of the phoneme :

post-aspirated as in top,
unaspirated as in stop.
glottalized as in button, but many speakers preserve at least an unreleased coronal stop.

In addition, the following allophones of /t/ are found in some dialects of American English;

flapped as in American English water,
nasal flapped as in American English winter.
unreleased as in American English cat, but other dialects preserve the released, or substitute the glottal stop.

However, speakers may become aware of the differences iffor examplethey contrast the pronunciations of the following words:

Night rate: unreleased
Nitrate: aspirated or retracted

A flame that is held in front of the lips while those words are spoken flickers more for the aspirated nitrate than for the unaspirated night rate. The difference can also be felt by holding the hand in front of the lips. For a Mandarin-speaker, for whom and are separate phonemes, the English distinction is much more obvious than for an English-speaker, who has learned since childhood to ignore the distinction.
One may notice the allophones of English such as the alveolar "light" of leaf as opposed to the velar alveolar "dark" in feel found in the U.S. and Southern England. The difference is much more obvious to a Turkish-speaker, for whom and are separate phonemes, than to an English speaker, for whom they are allophones of a single phoneme.
These descriptions are more sequentially broken down in the next section.

Rules for English consonant allophones

, a renowned phonetician, clearly explains the consonant allophones of English in a precise list of statements to illustrate the language behavior. Some of these rules apply to all the consonants of English; the first item on the list deals with consonant length, items 2 through 18 apply to only selected groups of consonants, and the last item deals with the quality of a consonant.
These descriptive rules are as follows:

Consonants are longer when they come at the end of a phrase. This can be easily tested by recording a speaker saying a sound like "bib", then comparing the forward and backward playback of the recording. One will find that the backward playback does not sound like the forward playback because the production of what is expected to be the same sound is not identical.
Voiceless stops are aspirated when they come at the beginning of a syllable, such as in words like "pip, test, kick". We can compare this with voiceless stops that are not syllable initial like "stop" . The voiceless stop follows the here.
Voiced obstruents, which include stops and fricatives, such as, that come at the end of an utterance like in "improve" or before a voiceless sound like in "add two") are only briefly voiced during the articulation.
Voiced stops and affricates in fact occur as partially devoiced at the beginning of a syllable unless immediately preceded by a voiced sound, in which the voiced sound carries over.
Approximants are partially devoiced when they occur after syllable-initial like in "play, twin, cue".
Voiceless stops are not aspirated when following after a syllable initial fricative, such as in the words "spew, stew, skew."
Voiceless stops and affricates are longer than their voiced counterparts when situated at the end of a syllable. Try comparing "cap" to "cab" or "back" to "bag".
When a stop comes before another stop, the explosion of air only follows after the second stop, illustrated in words like "apt" and "rubbed".
Many English accents produce a glottal stop in syllables that end with voiceless stops. Some examples include pronunciations of "tip, pit, kick".
Some accents of English use a glottal stop in place of a when it comes before an alveolar nasal in the same word, such as in the word "beaten".
Nasals become syllabic, or their own syllable, only when immediately following an obstruent, such as in the words "leaden, chasm". Take in comparison "kiln, film"; in most accents of English, the nasals are not syllabic.
The lateral, however, is syllabic at the end of the word when immediately following any consonant, like in "paddle, whistle".
# When considering as liquids, is included in this rule as well as present in the words "sabre, razor, hammer, tailor".
Alveolar stops become voiced taps when they occur between two vowels, as long as the second vowel is unstressed. Take for instance mainly American English pronunciations like "fatty, data, daddy, many".
# When an alveolar nasal is followed by a stop, the is lost and a nasal tap occurs, causing "winter" to sound just like "winner" or "panting" to sound just like "panning". In this case, both alveolar stops and alveolar nasal plus stop sequences become voiced taps after two vowels when the second vowel is unstressed. This can vary among speakers, where the rule does not apply to certain words or when speaking at a slower pace.
All alveolar consonants assimilate to dentals when occurring before a dental. Take the words "eighth, tenth, wealth". This also applies across word boundaries, for example "at this".
Alveolar stops are reduced or omitted when between two consonants. Some examples include "most people" (can be written either as or with the IPA, where the is inaudible, and "sand paper, grand master", where the is inaudible.
A consonant is shortened when it is before an identical consonant, such as in "big game" or "top post".
A homorganic voiceless stop may be inserted after a nasal before a voiceless fricative followed by an unstressed vowel in the same word. For example, a bilabial voiceless plosive can be detected in the word "something" even though it is orthographically not indicated. This is known as epenthesis. However, the following vowel must be unstressed.
Velar stops become more front when the following vowel sound in the same syllable becomes more front. Compare for instance "cap" vs. "key" and "gap" vs. "geese".
The lateral is velarized at the end of a word when it comes after a vowel as well as before a consonant. Compare for example "life" vs. "file" or "feeling" vs. "feel".