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The expression ‘nuclear structures’ has become, in our day, a term to conjure with; but the concept is not new in linguistics. It is mentioned or implied in contemporary discussions under the terms ‘immediate constituents’, ‘rank’, and ‘endocentric phrases’; in the older literature it is referred to as ‘modification’, ‘attribution’, or ‘subordination’. An assumption of different ranks is implicit in such word-pairs as stem-affix, head–attribute, noun-adjective, substantive–modifier, verb–adverb, principal-subordinate.
Richard S. Pittman (Wed,) studied this question.