This paper examines covariation in number specification in small clauses with two nominals, as exemplified by copular clauses with a nominal predicate (DP1 BE DP2). We show that number matching between DP1 and DP2 is obligatory in some such structures, but not others. Two questions arising from this are addressed: (A) By what formal mechanism should number matching between the DP1 and DP2 be enforced? (B) What determines whether matching is required or not? Regarding (A) we argue that Agree is not well-suited to modeling this feature-sharing relation and we propose instead that number matching arises as a reflex of feature-valuation at Merge when DP2 has an unvalued number feature [_#]. Regarding (B) we propose that the availability of valuation as a reflex of Merge is sensitive to the feature structure of DP2 and is blocked when, for independent reasons, DP2 does not introduce an accessible [_#]. We examine three such cases. In one case DP2 is referentially complete and has valued [#] rather than unvalued [_#]. In another case, DP2 is a predicate nominal with reduced functional structure (Persian). In the third case DP2 involves a concealed proposition.