Progressive place assimilation (PPA) implies that the coronal nasal consonant, /n/, adopts the place specification of a preceding consonant, examples of which are given in the following table:

Induced by a labial segment	Induced by a velar segment
koppen/kop+ən/[kopm̩]cups	sokken/sɔk+ən/[sɔkŋ̩]socks
lampen/lampə+ən/[lampm̩]lamps; bulbs	fisken/fɪsk+ən/[fɪskŋ̩]fishes
gaspen/gɔsp+ən/[ɡɔspm̩]buckles	lekken/lɛkən/[lɛkŋ̩]cloth, sheet
wapen/va:pən/[va:pm̩]weapon	ringen/rɪŋ+ən/[rɪŋŋ̩]rings
libben/lɪbən/[lɪbm̩]life; alive; lively	sangen/saŋən/[saŋŋ̩]purple
lammen/la:m+ən/[lamm̩]lambs
immen/ɪmən/[ɪmm̩]someone, somebody

There is one moving force behind both progressive and regressive place assimilation, viz. that /n/ must have the same place specification as the consonant it is adjacent to.

Be that as it may, there are considerable differences between the two kinds of place assimilation. In the first place, due to the fact that it can only occur in word-final position, the velar nasal /ŋ/ is one of the triggering consonants in progressive assimilation − see, for instance, ringen/rɪŋ+ən/[rɪŋŋ̩]rings and sangen/saŋən/[saŋŋ̩]purple in the right-hand column of the above table − where as it has no role to play in regressive assimilation.

In the second place, regressive and progressive place assimilation differ in the kind of /n/ they target: the former plain [n], the latter syllabic [n̩].

There are all kinds of indications that a syllabic sonorant consonant derives from the sequence schwa + sonorant consonant (see distributional evidence that syllabic consonants derive from /ə/ + consonant). In underlying representation therefore the preceding consonant and /n/ are separated by schwa (see the phonological representations in the table above). It is as the result of the deletion of schwa that /n/ and the consonant which precedes it become adjacent. Now, /n/ must have the same place specification as the consonant which it is adjacent to, which can only be achieved here by means of progressive assimilation. The resulting word-final consonant sequence violates the Sonority Sequencing Constraint (see onset: complex onsets). This violation is repaired by the projection of a syllable on top of /n/ or, put differently, by /n/ becoming syllabic [n̩].

In the third place, regressive place assimilation of /n/ is not triggered by a fricative − in fact, the latter enforces nasalization of the vowel preceding /n/ (see vowel nasalization). But a fricative does induce the kind of progressive place assimilation discussed here. As to this, however, there is a difference between the northern and the southern parts of the Frisian language area.

In the northern parts, syllabic [n̩] occurs after all coronal consonants, including the fricatives /s/ and /z/, as exemplified in (1):

The non-coronal fricatives do not trigger assimilation, as the forms in (2) make clear:

Take (te) draven/dra:v+ən/run (gerund). The realization [dra:vn] ‒ with plain [n] ‒ is out, because with respect to its place specification /n/ must be in harmony with its consonantal surroundings, which is not the case here. This demand on /n/ can be satisfied by progressive assimilation, resulting in the realization [*dra:v{ɱ/m̩}], with a syllabic labio-dental or bilabial nasal. In the northern parts of the language area, however, fricatives do no trigger this kind of assimilation. The upshot of this is that schwa must not be deleted, so that the fricative and /n/ end up as non-adjacent (which is also the case with the underlying representations of these forms).

Fricatives do not trigger assimilation in the northern parts of the language area, nor do they do this in the forms in (1) above. That a syllabic [n̩] is fine here is due to the fact that /n/ is in harmony with the preceding coronal consonant from the outset.

In the southern parts of the language area, there is assimilation of /n/ after all fricatives, see (3):

The final form ‒ (te) dragen/dra:ɣ+ən/[dra:ɡŋ]carry (gerund) ‒ deserves some comment. It contains the velar plosive [ɡ], which derives from the underlying fricative /ɣ/. It need not be assumed that /ɣ/ only turns into [ɡ] in the context of a syllabic /n/. In the southern parts, /ɣ/ is not allowed in intervocalic position, as shown by words like toga[to:ɡa]gown and lego[le:ɡo:]lego, which are pronounced with [ɣ] in the northern parts. So, a form like (te) dragen/dra:ɣ+ən/ turns into [dra:ɡən], which shows up as [dra:ɡŋ].