Assimilation is a process whereby adjacent consonants become more similar to each other (e.g. in manner or place of articulation) in order to facilitate pronunciation. Assimilation is the result of the process known as coarticulation that refers to the effect of the phonetic context on the articulation of speech sounds.
It refers to prolongation of the articulatory setting characteristic of a phoneme that occurs in the left context, so that the articulatory features of the phoneme in the right context are affected, e.g.
świat /ɕfjat/, krzywy /kʃyvy/, twarz /tfaʃ/
It occurs when the articulators are set in the position characteristic of a phoneme in the right context too soon. As a result the articulatory features of the phoneme in the left context are changed, e.g.
szybko /ʃɨpko/, arabski /arapsci/, babcia /bapɕa/
1.2Typology of assimilations in Polish
the direction of the effect of the phonetic context
progressive or perserverative a. (left to right: a sound changes with reference to a preceding segment)
regressive or anticipatory a. (right to left: a sound changes with reference to a following segment)
Assimilations concerning place or manner of articulation are mostly regressive and word-internal.
A stop may assimilate with respect to manner of articulation to the following fricative or affricate: this process is known as affrication. It occurs when the articulators are set to a position appropriate for the following sound too early. As a result the plosive acquires a fricative release.
Fricatives may assimilate regressively with respect to place of articulation: the tip of the tongue is set to a position characteristic of the following fricative before the friction of the first one is released.