Phonemes and allophones
Virtually all theories of phonology hold that spoken language can be broken down into a string of sound units, and that each language has a small, relatively fixed set of these sounds.
Take the words pit and bit for example and think about how just a single sound changes the meaning of the word. P and b are two distinctive sounds within the English sound system and are referred to as phonemes. Phonemes then are defined as the smallest distinctive or contrastive units of the sound system of a language.
By calling a sound distinctive, we refer to its capability of changing the meaning of a word. Naturally, single sounds cannot carry any meaning. "B" or "P", for example, are meaningless utterances. But when several distinct sounds are assembled to a word, each of them suddenly contributes to a meaning. And by exchanging individual distinct sounds, we may change this meaning.
Word pairs such as pit and bit, where one sound changes the meaning of the word, are called minimal pairs and you will find some exercises where you can check your perception and knowledge in terms of the smallest distinctive units of the English language below.
Most phonemes can be put into groups; for example, in English we can identify a group of plosive phonemes / p t k b d g /, a group of voiceless fricatives / f s h / and so on.
Source of the picture: mareni-gourmet.com
Phonemes are abstract sounds that we store in our mind and use for reference. They are ideal units of the sound system of a language and should not be confused with the sounds of actual utterances examined by phonetics. Phonetics tries to differentiate among the sounds with the highest possible degree of accuracy. It does so without regard for the influence a sound may have on the meaning of an utterance.
Each individual will pronounce the word "me" in a unique way, depending on the shape of the persons vocal tract or on whether someone is shouting or asking for their sixth martini. Still we (mostly) understand what is said and this has to do with the fact that we do not store all the possible ways of saying the word "me" or "I" in our brain, but an abstract version of speech sounds that are part of the language we speak.
Of course, not all sounds of a language are necessarily distinctive sounds. Take for instance the Let's call the whole thing off - video and think about how Ella Fitzgerald and Louis Armstrong pronounce words differently. Obviously they use different sounds when pronouncing the word tomato, the meaning of the word, however, does not change. Phonemes, the idealized, abstract sounds of a language, can be realized in many different ways and we call these different realizations of a phoneme allophones.
You know already, that phonemes can be identified through a minimal pair test. But how can you identify different allophones and, if you are not familiar with a language yet, how can you decide, whether two sounds are actually two separate phonemes or just allophones of one phoneme? You will learn more about this in the next subchapter.