Assimilation is the phonological process which makes two (or more) adjacent consonants more similar to each other than when they stand on their own or, put differently, assimilation is the process as a result of which two (or more) adjacent consonants come to share a phonological feature which they do not share when occurring in isolation.

In Frisian, the manner feature [±voice] and the place features [labial], [coronal], and [dorsal] are involved in the process, which is reflected in the terms voice assimilation and place assimilation, respectively. Furthermore, assimilation applies in one direction at a time, viz. it operates either from right to left or from left to right. The former mode of application is called regressive assimilation or anticipatory assimilation, the latter progressive assimilation or perseverant assimilation. Combining these two kinds of assimilation yields the following four types:

1. regressive place assimilation
2. regressive voice assimilation
3. progressive place assimilation
4. progressive voice assimilation

Place assimilation, regressive or progressive, targets one specific segment, viz. the coronal nasal /n/. As to voice assimilation, regressive assimilation is the general type − it targets segments which can be characterized in terms of broad phonological classes and it applies whenever the conditions of the context are met. Progressive voice assimilation, on the other hand, is the specific type − it only targets specific (single) segments which belong to specific suffixes or classes of words. Another difference is that regressive voice assimilation entails the voicing of segments, while the result of progressive voice assimilation is that segments become voiceless, i.e. they are devoiced. In all relevant respects then both types of voice assimilation contrast sharply with each other.

The feature [voice] is only distinctive within the class of the obstruents (see sonorants and obstruents and the features [±voice] and [±cont]) ‒ nasals, liquids, and glides are inherently voiced. This is likely to be the reason that it is only obstruents which are affected by voice assimilation. The latter can only have a phonetic effect on sonorants, whereas it has a neutralizing effect on obstruents.

Assimilation applies within a certain phonological domain. As to this, there is a neat division of labour between regressive and progressive assimilation. Regressive assimilation occurs between words (in a phonological sense): 1) between separate words in the context of a sentence, 2) between the members of a compound, 3) between prefixes and stems, and 4) between stems and those suffixes which count as words. Progressive assimilation occurs within phonological words. Here as well then both types of assimilation show a sharp contrast.

The counterpart of assimilation is dissimilation. It makes adjacent consonants less similar. In Frisian, dissimilation is much less common than assimilation. To be more precise, there is only one instance of it, concerning the voiceless final sequence /-xs/.