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boundary

MORPHOLOGY: a formal device used in Chomsky & Halle (1968) to express a distinction between two types of affixes. The assumption that affixes are associated with different boundaries, viz. + (morpheme boundary) and # (word boundary) accounts for the fact that the English suffixes -ity and -ness behave differently with respect to a number of phonological rules, as shown in (i): 

(i)	prodúctive   productívity  prodúctiveness
	op[ei]que    op[æ]city	   op[ei]queness
The claim is that -ity is a morpheme-boundary or formative-boundary affix (i.e. +ity), and -ness a word-boundary affix (i.e. #ness). See Level Ordering Hypothesis.
LIT. Chomsky, N. and M. Halle (1968)