#Manifestation of phonemes in speech. Phoneme and allophone.
Allophones of a certain phoneme are speech sounds which are realizations of one and the same phoneme and which can't distinguish words.
Allophones can be:
1. Positional - are used in certain positions traditionally; can be dialectal or individual.
2. Combinatory - appear in the result of assimilations, adaptation, accomodation.
Allophones of the same phoneme, function as the same linguistic unit. Phonemes differentiate words from each other, and to be able to hear and produce phonemic differences is a part of what it means to be a competent speaker of the language. Allophones, have no such function: they usually occur in different positions in the word and hence can't be opposed to each other to make meaningful distinctions.