Speakers consistently recognize and interpret physically different sounds as the same sound. It is practically impossible to pronounce words, even the same word, in exactly the same way twice, an incredible variety of speech sounds reaches our ears every day. The size and shape of the vocal tract varies from one speaker to the next, and since speech sounds derive from anatomical functions, different anatomies produce different sounds. The knowledge of a sound and its variant are part of the speakers' phonological competence . A mental unit of speech is a phoneme .
Phonemes are contrastive sound units which make a difference in words.
Time, Dime, Rhyme, Lime, Chime, and Mime are a minimal set. Time and Dime are a minimal pair. A physical sound unit is an allophone . Allophones are non-contrastive. They may also offer an explanation of why the process takes place.