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In phonetics, (a branch of linguistics) a phone is any distinct speech sound or gesture, regardless of whether the exact sound is critical to the meanings of words.
In contrast, a phoneme is a speech sound in a given language that, if swapped with another phoneme, could change one word to another. Phones are absolute and are not specific to any language, but phonemes can be discussed only in reference to specific languages.
For example, the English words kid and kit end with two distinct phonemes, /d/ and /t/, and swapping one for the other would change one word into a different word. However, the difference between the /p/ sounds in pun ([pʰ], with aspiration) and spun ([p], without aspiration) never affects the meaning or identity of a word in English. Therefore, [p] cannot be replaced with [pʰ] (or vice versa) and thereby convert one word to another. That causes [pʰ] and [p] to be two distinct phones but not distinct phonemes in English.
In contrast to English, swapping the same two sounds in Hindustani changes one word into another: [pʰal] (फल/پھل) means 'fruit', and [pal] (पल/پل) means 'moment'.[1] The sounds [pʰ] and [p] are thus different phonemes in Hindustani but are not distinct phonemes in English.[2]
As seen in the examples, phonemes, rather than phones, are usually the features of speech that are mapped onto the characters of an orthography.
In the context of spoken languages, a phone is an unanalyzed sound of a language.[3] A phone is a speech segment that possesses distinct physical or perceptual properties and serves as the basic unit of phonetic speech analysis. Phones are generally either vowels or consonants.
A phonetic transcription (based on phones) is enclosed within square brackets ([ ]), rather than the slashes (/ /) of a phonemic transcription, (based on phonemes). Phones (and often also phonemes) are commonly represented by using symbols of the International Phonetic Alphabet (IPA).
For example, the English word spin consists of four phones, [s], [p], [ɪ] and [n] and so the word has the phonetic representation [spɪn]. The word pin has three phones. Since its initial sound is aspirated, it can be represented as [pʰ], and the word's phonetic representation would then be [pʰɪn]. (The precise features shown in a phonetic representation depend on whether a narrow or broad transcription is used and which features the writer wishes to draw attention to in a particular context.)
When phones are considered to be realizations of the same phoneme, they are called allophones of that phoneme (more information on the methods of making such assignments can be found under phoneme). In English, for example, [p] and [pʰ] are considered allophones of a single phoneme, which is written /p/. The phonemic transcriptions of those two words is thus /spɪn/ and /pɪn/, and aspiration is then no longer shown since it is not distinctive.
Whether a direct mapping between phonemes and characters is achieved depends on the type of orthography used, phonological orthographies like the Indonesian orthography tend to have one-to-one mappings of phonemes to characters whereas alphabetic orthographies like the English orthography tend to try to have direct mappings but end up mapping one phoneme to multiple characters often.
In the examples above the characters enclosed in square brackets: "pʰ" and "p" are IPA representations of phones. The IPA unlike English and Indonesian is not a practical orthography and is used by linguists to obtain phonetic transcriptions of words in spoken languages and is therefore a strongly phonetically spelled system by design.