In this paper it is proposed that a basic difference in structural dependency, the difference between selected complements of a head on the one hand and unselected adjuncts to a projection of a head on the other, correlates with a basic interpretive difference which is central to understanding the range of complex predicate constructions. Adjuncts, such as depictive predicates, are interpreted as conjuncts, as is commonly assumed. The standard assumption about complements is that their interpretation is by function application, where the specifics are then determined by the semantics of the selecting head. Here it is proposed that there is a common interpretive core to complementation, one which is monotonic and in some abstract sense mereological. This applies both to extended projections and to complements which are embedded in another extended projection, and is exemplified by resultatives as well as other secondary predicate types. The interpretation is monotonic in the sense that the selecting head preserves the interpretation of the complement, elaborating on it. It is mereological in the related sense that the complement is interpreted as an integral part of the interpretation of the whole.