This paper addresses the following questions: is (external) merge, the binary operation that combines two elements into a constituent in every variant of the Minimalist Program (Chomsky, 1993, 1995 and related works), an unconstrained operation? If so, what avoid generating ill-formed structures? I will argue here for a simple functional / lexical constraint on Merge, assuming a possible principled binary opposition for the items which enter the syntactic derivation. I will basically follow Kayne (2009), who assumes that the class of nouns (or L-roots) is the only open (lexical) class in grammar, updating the intuitions of Hale and Keyser (1993). This proposal leads to interesting structural and typological consequences.