Vowel harmony
Vowel harmony is usually controlled by the root of a word, so that the vowels of other morphemes assimilate to the root vowels' tongue root position. However, some suffixes are "strong" and instead assimilate the root along with any preceding suffixes. Prefixes are always weak and do not control other vowels. The vowels paired in such assimilations are /i/ vs. /ɪ/, /e/ vs. /ɛ/, /o/ (from earlier /ə/) vs. /a/, /o/ vs. /ɔ/, and /u/ vs. /ʊ/; either element of each pair will turn into the other to match the tongue root position of the controlling morpheme. Vowel harmony does not cross word boundaries, and a phonological word can be defined as a unit across which harmony operates. Vowel harmony is also blocked at the boundary between roots in a compound.