To know how sounds are produced is not enough to describe and classify them as language units. When we talk about the sounds of language, the term "sound" can be interpreted in two different ways. First, we can say that [t] and [d], for example, are two different sounds in English: e.g. ten-den, seat-seed. But on the other hand, we know that [t] in let us and [t] in let them are not the same. In both examples the sounds differ in one articulatory feature only. In the second case the difference between the sounds has functionally no significance. It is clear that the sense of "sound" in these two cases is different. To avoid this ambiguity, linguists use two separate terms: phoneme and allophone. Allophones are variants of phonemes.
Shcherba, the head of Leningrad linguistic School stated that in the spoken language a much greater number of various sounds are pronounced than we usually think and these sounds in every language are united in a comparatively small number of sound types, which are capable of distinguishing the meaning and the form of words; i.e. they serve the purpose of social intercommunication.
Such sounds he called PHONEMES. The actually pronounced speech sounds are variants or ALLOPHONES.
Allophones are realized in concrete words. They have similarity from the phon. point of view, i.e. the acoustic and articulatory pitches have much in common. At the same time they differ in some degree and are incapable of differentiating words. Ex, in speech we pronounce not the sound type /t/ which is alveolar, forelingual, apical, occlusive, plosive, voiceless, strong, according to the classificatory definition, but one of its variants. For ex, labialized in the word "twice", dental in the word "eighth", post-alveolar in "try", exploded nasally in "written" and exploded literary in "little".
Another ex. the sound type /i:/ which is defined as unrounded, fully-front, high, narrow, long, free is more back in the word KEY than in EAT under the influence of the back-lingual /k/ it is longer before a voiced consonant than before a voiceless. Ex., SEED-SEET-GREED-GREET.
The number of sound types or phonemes in each language is much smaller than the number of allophones. Allophones are very important for language teaching because they are pronounced in actual speech and through their mispronunciation doesn't always influence the meaning of the words their misuse makes a person's speech sound as "foreign".
The phoneme is a minimal abstract linguistic unit realized in speech in the form of speech sounds opposable to other phonemes of the same language to distinguish the meaning of morphemes and words.
Phonetics and other branches of linguistics | The theory of the phoneme in its historical development | The theory of the phoneme in foreign phonological schools | The distinctive and constitutive functions of segmental phonemes | Classification of Eng. vowels | Phonological analysis of English vowels | The articulatory aspect of Eng. vowels | Classification of Eng. cons-s | Phonological analysis of English consonants. | The articulatory aspect of Eng. cons-s |