The assertion that a clitic and its host word form one phonological word presupposes that there are criteria for establishing the clitichood of a function word and, by the same token, the phonological wordhood of the combination of a function word and its host word. The question is therefore when the latter combination can be said to form a phonological word or, put differently, when a function word can be said to show clitic behaviour. As to this, Gussenhoven (1985-1986:180) formulates the, slightly rephrased, criterion:

It does not seem too far-fetched to equate the unit for which phonological conditions hold which do not hold for combinations of two non-function words with the phonological word.

Taking the above criterion as a point of departure, there appears to be evidence for the clitic behaviour of function words in Frisian. In the first place, there is evidence based on syllabification. Take the examples below:

The phonological word is considered to be the domain of syllabification. In the examples in (2a), the article it /ət/ the (with neuter nouns) makes up a single phonological word with, viz. cliticizes onto, the prepositions oan /oən/ on and oer /uər/ over by which it is preceded. The syllabification of oan it /oən ət/ and oer it /uər ət/ is as in the example below:

The final segments of the preposition − /n/ and /r/− form the onset of the right-most syllable, in line with the demand that a word-internal schwa syllable should have an onset (see Schwa). Since these prepositions have a long vocalic sequence here − the centring diphthongs /oə/ and /uə/−, the left-most syllable is in line with the Rhyme Constraint, according to which the rhyme of a word-internal syllable must occupy two structural positions. As to syllabification then oan it (dak) and oer it (wetter) behave just like the inflected words toanen /toən+ən/ [(toə)(nən)] tones and boeren /buər+ən/ [(buə)(rən)] farmers.

The syllabification of op it and fan it in (2b) is as below:

Due to short vowels of the prepositions, their final segments − /p/ and /n/− end up as ambisyllabic, that is, as both the coda of the first and the onset of the second syllable, simultaneously satisfying the onset demand for word-internal schwa syllables and the Rhyme Constraint. In this respect, the above sequences resemble the inflected words koppen /kop+ən/ [(kop)(pən)] cups and sokken /sɔk+ən/ [(sɔk)(kən)] socks.

Final Devoicing, in the second place, also gives us a clue. Its effect on the realization of word-final plosives in the combination of a finite verb and a personal pronoun are exemplified below:

It seems to be clear that the final obstruents of the above verbs have undergone Final Devoicing. However, the syllabification of ried ek and hold ek − [(riə)(tək)] and [(ho:)(tək)], respectively − is such that verb-final /d/ ends up as the onset of the right-hand syllable, a position in which it cannot be devoiced. Devoicing thus must have taken effect here at a stage where /d/ was in coda position, i.e., when the personal pronoun ek /ək/ I had not yet been amalgamated with the verb. If the phonological word is the domain of syllabification, it is also the domain of resyllabification. Invoking resyllabification here seems to be the only way to reconcile /d/'s underlying voicedness, its devoicing in coda and its voicelessness in onset position. Devoiced /d/ in onset position thus also testifies to the the clitic behaviour of function words. See Booij (1995:174-176) for the same phenomenon in Dutch.

One and the same stem may be operated upon by inflection/derivation on the one and cliticization on the other hand; this may give rise to 'minimal pairs', examples of which are given below:

The primeless examples are realized with [d], the voicing value of which matches that of the /d/ of the underlying representations hoeden /huədən/ cautious, wurden /vød+ən/ words, and rieder /riəd+ər/ advisor. These words are syllabified as [(huə)(dn̩)], [(vød)(dn̩)], and [(riə)(dr̩)], respectively, in line with both the Rhyme Constraint and the onset demand for word-internal schwa syllables; the syllabic sonorant consonants, [n̩] and [r̩], derive from the sequences /ə{n/r}/ (see Evidence that syllabic consonants derive from /ə/ + consonant). Clearly, syllabification takes the underlying representations of these words as its point of departure. The primed examples consist of the same rows of segments as the primeless ones, but they accommodate different morpho-syntactic entities, viz. a sequence of words − like wurd in word a − versus one (complex) word − as with wurden words. If it is assumed that words are juxtaposed at the syntactic level after they have been operated upon by word-level phonology, among which Final Devoicing, it is explained why underlying /d/ is realized as [t] here (see Booij (1995:174-175)) for the same analysis of comparable Dutch cliticization facts). Apart from the difference between [d] and [t], the syllabification of the primed word sequences − [(huə)(tn̩)], [(vøt)(tn̩)], and [(riə)(tr̩)] − equals that of the primeless (complex) words, also with respect to the above demands. As with the examples in (5), devoiced /d/ in onset position is indicative of the the clitic behaviour of function words, in this case in /ən/ a(n) and er /ər/ he.

In the third place, word-level phonological processes are expected to apply within the combination of a host word and a clitic. The occurrence of syllabic sonorant consonants is a case in point (see Syllabic sonorant consonants). This is exemplified below (see also the cases in (6) above):

The above combinations of host word and clitic are syllabified as [(hat)(tr̩)], [(rɔ:)(pm̩)] and /(op)σ(pm̩)σ/: the final segment of hat /hat/ has, rôp /rɔ:p/ called, and op /op/ on has undergone resyllabification ((8b)) or ambisyllabification ((8a,c)), in line with a) the Rhyme Constraint and b) the demand that a word-internal schwa syllable have an onset (note again that a schwa syllable underlies the syllabic sonorant consonants). The occurrence of the syllabic sonorant consonants therefore also testifies to the the clitic behaviour of function words.

Finally, a phonological word constitutes an uninterruptable unit. This also holds of the combination of a host word and a clitic, as is illustrated below:

If a content word and a function word are separated by a pause ((10a,b)) or a parenthesis ((10c,d)), only the full form of the function word is allowed. As to the above cases this means that hy /hi/ he, de /də/ the, sij /sɛj/ she, and jim /jɪm/ you (object form, plural, familiar and polite) are allowed, to the exclusion of their clitic counterparts er /ər/, 'e /ə/, se /sə/, and jem /jəm/. Cases like those in (10) therefore lend support to the assertion that a clitic and its host word form one phonological word or, the other way around, they testify to the the clitic behaviour of the above function words.