Vowel Harmony
In the native lexis, any vowel except /ə/ can occur in morpheme-final syllables, while /ə/ predominates in antepenultimate syllables. Only /i/, /u/ and /a/ occur in final open syllables. Vowel sequences occur only across syllable boundaries, and there are vowel harmony rules within a morpheme (see below). /ə/ never occurs immediately before a following vowel; where /ə/ is penultimate, only /i/, /u/ or /a/ may occur in the final syllable.
A rule of vowel harmony applies morpheme-internally to the non-central vowel phonemes (/i/, /u/, /e/, /o/). Where they occur in both the penultimate and final syllables, they must agree in height (but not necessarily in frontness), thus we find giling /giliŋ/ 'mill' and burung /buruŋ/ 'bird', and also words like hidung /hiduŋ/ 'nose' and boleh /boleh/ 'can, may'. Sequences of high and mid non-central vowel phonemes do not occur in either order, thus the following do not occur: */heduŋ/, */bolih/, and (at the phonemic level) */hidoŋ/, */gileŋ/ (Adelaar 1992).