P in the surface phonology. There have been other, relation-based, proposals concerning the nature of syntax–prosodic structure interface constraints as well, including Nespor and Vogel 1986, McHugh 1990, Kim 1997, all understood to be consistent with the prosodic structure hypothesis, and with the input–output architecture. An input–output model of the syntax–phonology interface makes the essential claim that constraints on phonological output representation do not interact with constraints on the surface syntactic representation. Two sorts of apparent challenges for the input–output model have recurred in recent literature. One sort concerns clitics whose distribution is governed by both syntactic and prosodic constraints, such as the second position clitics of Serbo-Croatian (Inkelas and Zec 1990). Yet because the syntax itself affords options in the positioning of second position clitics, it is not necessary to construe the prosodic subcategorization of a clitic as outranking a constraint on syntactic word order. Another sort of challenge concerns the alleged dependence of word order on the positioning of focus-related prosodic prominences in the sentence, as suggested by Vallduvi (1991) for Catalan, or Zubizarreta (1998) for Spanish. However, since the latter sort of phenomenon potentially can receive a purely syntactic treatment in which the Focus properties of syntactic representation are crucial in determining word order (see, e.g., Grimshaw and Samek-Lodovici 1998), this sort of case is not yet compelling. It remains to be seen whether the syntax– phonology interface is an input–output relation or whether it is instead a two-way street.