In this paper we examine clitic placement in Medieval Spanish (Medsp) as well as the Person Case Constraint (PCC) arguing that a natural explanation for both these phenomena can be given once we assume clitics to be the encoding of calcified processing strategies of an earlier freer word order system (Kempson and Cann, 2007; Bouzouita, 2008a,b,c; Chatzikyriakidis and Kempson, 2009; Chatzikyriakidis, forthcoming). We show that the availability of different parsing strategies being possible for one and the same string, led to cases where re-analysis in terms of the parser gave rise to syntactic change. Assuming that each clitic in effect matches one of the four different parsing strategies of the earlier Latin scrambling system, the PCC facts are straightforwardly accounted. Assuming that syncretized and dative clitics involve the projection of an unfixed node with no form of update, any combination of first/second clitics or a third dative plus a 1st/2nd clitic is predicted to be illicit by a very general constraint on tree-growth, the fact that no more than one unfixed node with the same underspecified address can be present in the tree structure, since by definition these two will collapse into one by means of treenode identity.