Schwa epenthesis can affect onset sequences. Sipma (1913:14, §50) makes some general remarks on the realization of the Frisian consonants. He observes that word-initial consonants or the second member of an initial cluster, provided it is a sonorant consonant, can become lengthened through emphasis. Some of Sipma's examples are given in (1):

Sipma states in a footnote: In cases like these one also hears such forms as səmǒɑrg̊, ɡəri:z̊, ɡəlö.ən, with insertion of the indistinct vowel ə between two consonants, when the second of them is a liquid.

We might ask whether we are dealing here with phonetic implementation instead of phonology proper or, put differently, whether schwa insertion is anything more than a side effect of emphasis, with no effects on the phonological structure. The more so since epenthesis takes place in the unmarked complex onset of obstruent plus liquid, which does not seem to be in need of an inserted vowel at all.

It should be taken into account, however, that epenthesis never affects complex segments (of the form /s/ + plosive or plosive + /s/) − which are a linked structure (see complex segments as single units). Words like spul/spøl/gear, things; quarrel, jar, stil/stɪl/silent, and skek/skɛk/(skating) stroke cannot be realized as [*səpøl], [*sətɪl], and [*səkɛk], in whichever style of speech. Phonological structure thus does have a role to play in accounting for schwa insertion in onset sequences.

Schwa insertion affects onset sequences of the form obstruent plus liquid, see the examples in (2):

It also affects onset sequences consisting of an extrasyllabic segment and a sonorant consonant, as in the examples in (3):

Schwa insertion does not affect a complex onset in every position within the word. In prefixed words, like those in (4), insertion yields an ill-formed outcome:

Schwa insertion would give rise to a lapse here − a disfavoured rhythmic configuration −, which may be reason for its non-occurrence. In a form like ûntbleat(sje)/unt+blɪət/to bare, to strip; to expose, with the full vowel prefix ûnt- (/unt-/), the result is acceptable: [undblɪətsjə], [undbəlɪətsjə].