PHONEMES
Phonemes are the linguistically contrastive or significant sounds (or sets of sounds) of a language. Such a contrast is usually demonstrated by the existence of minimal pairs orcontrast in identical environment (C.I.E.). Minimal pairs are pairs of words which vary only by the identity of the segment (another word for a single speech sound) at a single location in the word (eg. [mæt] and [kæt]). If two segments contrast in identical environment then they must belong to different phonemes. A paradigm of minimal phonological contrasts is a set of words differing only by one speech sound. In most languages it is rare to find a paradigm that contrasts a complete class of phonemes (eg. all vowels, all consonants, all stops etc.).
eg. the English stop consonants could be defined by the following set of minimally contrasting words:-
i) /pɪn/ vs /bɪn/ vs /tɪn/ vs /dɪn/ vs /kɪn/
ALLOPHONES
Allophones are the linguistically non-significant variants of each phoneme. In other words a phoneme may be realised by more than one speech sound and the selection of each variant is usually conditioned by the phonetic environment of the phoneme. Occasionally allophone selection is not conditioned but may vary form person to person and occasion to occasion (ie.free variation).
A phoneme is a set of allophones or individual non-contrastive speech segments. Allophones are sounds, whilst a phoneme is a set of such sounds.
Allophones are usually relatively similar sounds which are in mutually exclusive or complementary distribution (C.D.). The C.D. of two phones means that the two phones can never be found in the same environment (ie. the same environment in the senses of position in the word and the identity of adjacent phonemes). If two sounds are phonetically similar and they are in C.D. then they can be assumed to be allophones of the same phoneme.
eg. in many languages voiced and voiceless stops with the same place of articulation do not contrast linguistically but are rather two phonetic realisations of a single phoneme (ie. /p/=[p,b],/t/=[t,d], and /k/=[k,ɡ]).
Linguistics 