tion is a common explanation of consonant changes in English. It frequently produces voicing or unvoicing, as in the change from /-z/ to /-s/ after voiceless consonants.
Another common change due to assimilation is that of /n/ to /m/ before bilabial consonants. Thus, the negative prefix {in-} has an allomorph /im-/ which occurs before /p, b/, as in impossible and imbalance. Similarly, the prefix {en-} has an allomorph {em-} occurring before /p, b/ in empower and embitter. The word imperfect contains a root and a prefix whose base form may be given as {in-}. The change of /n/, an alveolar nasal, to /m/, a bilabial nasal, makes it more similar to /p/, a bilabial stop. The assimilation of /n/ is said to be conditioned by /p/ (Gleason, 1955, p. 83).
Assimilation can be progressive: the assimilated sound follows the conditioning sound; or regressive: the assimilated sound precedes the conditioning sound. Assimilation conditioned by an immediately adjacent sound is sometimes called contiguous in contrast to noncontiguous, in which one or more phonemes intervenes
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