<P> Within linguistics there are differing views as to exactly what phonemes are and how a given language should be analyzed in phonemic (or phonematic) terms . However, a phoneme is generally regarded as an abstraction of a set (or equivalence class) of speech sounds (phones) which are perceived as equivalent to each other in a given language . For example, in English, the k sounds in the words kit and skill are not identical (as described below), but they are distributional variants of a single phoneme / k / . Different speech sounds that are realizations of the same phoneme are known as allophones . Allophonic variation may be conditioned, in which case a certain phoneme is realized as a certain allophone in particular phonological environments, or it may be free in which case it may vary randomly . In this way, phonemes are often considered to constitute an abstract underlying representation for segments of words, while speech sounds make up the corresponding phonetic realization, or surface form . </P> <P> Phonemes are conventionally placed between slashes in transcription, whereas speech sounds (phones) are placed between square brackets . Thus / pʊʃ / represents a sequence of three phonemes / p /, / ʊ /, / ʃ / (the word push in standard English), while (phʊʃ) represents the phonetic sequence of sounds (ph) (aspirated p), (ʊ), (ʃ) (the usual pronunciation of push). This is not to be confused with the similar convention of the use of angle brackets to enclose the units of orthography, namely graphemes; for example, ⟨ f ⟩ represents the written letter (grapheme) f . </P> <P> The symbols used for particular phonemes are often taken from the International Phonetic Alphabet (IPA), the same set of symbols that are most commonly used for phones . (For computer typing purposes, systems such as X-SAMPA and Kirshenbaum exist to represent IPA symbols using only ASCII characters .) However, descriptions of particular languages may use different conventional symbols to represent the phonemes of those languages . For languages whose writing systems employ the phonemic principle, ordinary letters may be used to denote phonemes, although this approach is often hampered by the complexity of the relationship between orthography and pronunciation (see Correspondence between letters and phonemes below). </P> <P> A phoneme is a sound or a group of different sounds perceived to have the same function by speakers of the language or dialect in question . An example is the English phoneme / k /, which occurs in words such as cat, kit, scat, skit . Although most native speakers do not notice this, in most English dialects the "c / k" sounds in these words are not identical: in kit (help info) (khɪt) the sound is aspirated, while in skill (help info) (skɪl) it is unaspirated . The words therefore contain different speech sounds, or phones, transcribed (kh) for the aspirated form, (k) for the unaspirated one . These different sounds are nonetheless considered to belong to the same phoneme, because if a speaker used one instead of the other, the meaning of the word would not change: using the aspirated form (kh) in skill might sound odd, but the word would still be recognized . By contrast, some other sounds would cause a change in meaning if substituted: for example, substitution of the sound (t) would produce the different word still, and that sound must therefore be considered to represent a different phoneme (the phoneme / t /). </P>

The various ways that a phoneme are pronounced are called allophones. an example of this is