The clitic string as a Pair Merge sequence.

We argue that clitics are structured by Pair Merge, rather than by Set Merge. This contrasts with classical approaches treating the head status of clitics as derived from a fundamentally phrasal syntax. We show that different structures of merger for clitics and phrasal arguments (including full pronouns) can explain well-known empirical differences that otherwise need to be stipulated or derived via extra-syntactic—viz. morphological—mechanisms. This view of cliticization seems to run into immediate problems once we move away from object clitics in finite declarative sentences in Italian or French and tackle slightly less familiar linguistic systems that allow interpolation, enclisis (inversion), and allow clitic sequences to split (object vs. subject clitics). We argue that the above phenomena receive a principled explanation under the assumption that clitics are pair merged with different phase heads.