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. The explanation lies in the phenomenon of assimilation , the natural process by which a sound changes in order to become more like another sound. We anticipate a sound and it changes another sound. Vowels become nasalized because they assimilate to the nasality of a following nasal. FACT 9 Phonological rules describe the regularities of the sound system of a language.