Underlying segment - a segment which constitutes a part of the underlying representation of a morpheme. It consists of a cluster of distinctive features. The obligatory element of a syllable is nucleus , a vowel or a syllabic consonant. Languages differ in what types of segments may constitute a nucleus and in the forms of the syllabic onsets and codas.
Assimilation is driven by articulatory constraints .
Dissimilation - when one segment becomes different from a preceding or a following segment. Dissimilation is driven by phonological motivation to distinguish between the segments. In English, some people say aks for ask . This is another example of metathesis. This happens to vowels in unstressed position and to consonants in word final position, or in a consonant cluster.