Abstract.  An extended noun phrase may contain an expression that describes some dimension. Weight is described by each of the prenominal expressions in heavy rock, too much ballast, 2 pound rock, 2 pounds of rocks. The central claim of this paper is that the position of these types of expressions within the noun phrase limits the kinds of dimensions they may describe. The limitations have to do with whether or not the dimension tracks relevant part-whole relations. An analogy is made between these constraints and the well-known constraints on thematic relations that are incurred by the position of a noun phrase in a clause. A proposal is made about the meanings of expressions like too much and 2 pounds which explains their common cross-categorial distribution and this informs the analysis of their use in noun phrases. A position is taken on the meaning of the count mass distinction which, in conjunction with the hypothesis about dimensions, explains asymmetries in the distribution of prenominal adjectives with count and mass nouns.