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In spoken language, a phoneme is a basic, theoretical unit of sound that can change the meaning of a word. A phoneme may well represent categorically several phonetically similar or phonologically related sounds (the relationship may not be so phonetically obvious, which is one of the problems with this conceptual scheme).

Depending on the language and the alphabet used, a phoneme may be written consistently with one letter; however, there are many exceptions to this rule (especially in English).

English has 40 phonemes, which is above average across all languages. Polynesian has 11, Khoisan has 140.

When representing phonemes in linguistic writing, it is common to use 'slash' markers as quotes around the symbol that stands for the sound. For example, the phoneme for the initial consonant sound in the word "phoneme" would be written as /f/. In other words, the English grapheme is , but this digraph represents one sound /f/. Allophones, real speech variants of a phoneme, are often denoted in linguistics by the use of diacritical or other marks added to the phoneme symbols and then placed in square brackets [ ] to differentiate them from the phoneme in slant brackets / /. The conventions of orthography are then kept separate from both phonemes and allophones by the use of the markers < > to enclose the spelling.

The symbols of the international phonetic alphabet (IPA) and extended sets adapted to a particular language are often used by linguists to write phonemes, with the principle being one symbol=one categorical sound. However, there is an augmented set for writing the IPA exclusively in plain text, and it is these conventions which are used in this article.

Examples of phonemes in the English language would include sounds from the set of English consonants, like /p/ and /b/. These two are most often written consistently with one letter for each sound. However, phonemes might not be so apparent in written English, such as when they are typically represented with combined letters, called digraphs, like (= SAMPA /S/) or (= SAMPA /tS/).

Phonology, or more specifically, phonemics, is the study of the system of phonemes of a language, although some conceptualize phonology as encompassing far more than sound segments. Thus phonology can be used as a more general term subsuming phonemics.

What may be an allophone (a sound variant belonging to the same phoneme category) in one language may be a phoneme itself in another language. In English, for example, [p] has aspirated and non-aspirated allophones, e.g. aspirated in /pIn/, but non-aspirated in /spIn/. However, in some languages (e.g., Ancient Greek), aspirated /ph/ was a phoneme distinct from both unaspirated /p/ and /b/. As another example, there is no distinction between /r/ and /l/ in Japanese, there is only one /r/ phoneme in Japanese, although the Japanese /r/ has allophones that make it sound more like an /l/ or /d/ to English speakers. The sounds /z/ and /s/ are distinct phonemes in English, but allophones in Spanish. /dZ/ (as in ) and /Z/ (as in , ) are phonemes in English, but allophones in Italian.

A sound that is a single phoneme in one language may be a phoneme cluster in another. For instance, /buts/ means leg-covering footwear in English and consists of four phonemes /b u t s/; but in Hebrew it means a kind of cloth and consists of only three phonemes /b u ts/.

The phoneme is a structuralist abstraction that was later adapted to and formally psychologized in generative linguistics (after Chomsky and Halle). Rather than a basic mental unit of language, however, it may well be a perceptual artifact of alphabetic literacy (see the terms Phonemic awareness and Phonological awareness). If not that, it may be an epiphenomenal aspect to listening removed from face-to-face encounters, that is, text-like listening. Cf. Phone and Feature.

Phonological extremes

Of all the speech sounds that a human vocal tract can create, different languages vary considerably in the number of these sounds that they consider to be distinctive. Some dialects of Abkhaz have only 2 vowels, and many Native American languages have 3, while Punjabi has over 25. Rotokas (spoken in Papua New Guinea) has only 6 consonants, while !Xu~ (spoken in southern Africa, in the vicinity of the Kalahari desert) has over 100. The total number of phonemes in languages varies from as few as 11 in Rotokas and 13 in Hawaiian to as many as 141 in !Xu~. These may range from familiar sounds like [t], [s] or [m] to very unusual ones produced in extraordinary ways (see: Click consonant, phonation, airstream mechanism). The English language itself uses a rather large set of 13 vowels, though its 27 consonants are pretty close to average. This differs from the lay definition based on the Latin alphabet, where there are 21 consonants and 5 vowels (although sometimes y and w are included as vowels).

The most common vowel system consists of five vowels: /i/,/e/,/a/,/o/,/u/.
The most common consonants are /p/,/t/,/k/. Not all languages have these; the Hawai'ian language lacks /t/, and the Mohawk language lacks /p/, but all known languages have at least two of the three. If one of the three is missing, the language will have /'/ (glottal stop).
Possibly the rarest sound is the one represented by the "ř" letter (called "r hacek") (found in the name Dvořak) in the Czech language; it appears to be unique to the language.

Only the Dyirbal language of Australia uses six (primary or contrastive) places of articulation; all other languages use fewer. The possible places of articulation include bilabial, labiodental, dental, alveolar, alveopalatal, palatal, retroflex, velar, uvular, pharyngeal, and glottal.

A language where one letter represents only one phoneme and one phoneme is representing only a letter, is called phonetic idiom, like Esperanto or Serbian.

See also minimal pair, initial-stress-derived noun.