Schwa is not allowed as the only vowel of a (content) word, so a vowel cannot undergo reduction if it is the only vowel of such a word. A canonical Frisian word has one full vowel, which may be supplemented by schwa (so it ends up as either mono- or disyllabic). In principle then vowel reduction can only manifest itself in loan words, which can have more than one (unstressed) full vowel.

There are, however, some exceptions to the above. First, compounds (in a broad sense) consist of two or more independent words, so vowel reduction is impossible. A compound, however, can develop meanings so remote from the original, compositional meaning that it comes to function as a simplex word. The implication of this is that it is exposed to phonological processes typical for words, one of them being vowel reduction. Examples of the latter in compounds with a no longer transparant meaning are given in (1):

Secondly, phrases can develop into words − in which case they tend to be written as one word − and can then undergo vowel reduction; examples are given in (2):

Thirdly, in fixed collocations consisting of two elements conjoined by en/ɛn/and, the full vowel /ɛ/ of the conjunction can reduce, of which (3) gives some examples:

Vowel reduction is not an obligatory process, that is, not every short vowel in unstressed position is affected by it. This may be a word-specific matter. The word fakânsjeholiday(s), for instance, is realized as [fə'kɔ̃:sjə] − [fa'kɔ̃:sjə] sounds weird −, whereas kabaalracket can be pronounced as both [ka'ba:l] and [kə'ba:l]. There are also contexts in which vowel reduction (virtually) does not occur, as in word-initial and word-final position (see vowel reduction in word-final position). In other contexts, it can and cannot occur, which is dependent on the concerted influence of several factors. These are mentioned in the list below. However, an explicit account of their relative contribution to the occurrence or non-occurrence of vowel reduction cannot be given, since this will have to be examined in more detail.

1. There is a rhythmic tendency in the language to alternate stressed and unstressed syllables, so clashes and lapses are avoided. One therefore expects vowel reduction not to occur in case it results in a lapse.
2. More frequently used words are expected to show more vowel reduction than less frequently used ones.
3. Vowel reduction is expected to occur more in casual than in formal speech (details about the influence of speech style on vowel reduction in Dutch can be found in The influence of speech style on vowel reduction).
4. Vowel reduction is expected to occur more in speech at a higher than in speech at a lower speech rate.
5. Vowel reduction is expected to occur less at morpheme boundaries than inside morphemes.
6. Not all vowels are equally prone to reduce. As to Dutch, for instance, the following hierarchy is assumed (see Booij (1995:134)):
   1. the vowels which reduce most easily are the ones closest to schwa: /e/ and /ɛ/.
   2. the vowel /a/ also reduces quite easily.
   3. rounded vowels reduce less easily than unrounded ones.
   4. the closed vowels reduce least easily.
   5. diphthongs never reduce.