Kotek, Hadas and Michael Yoshitaka Erlewine, 2016. “Covert pied-piping in English multiple wh-questions.”
Linguistic Inquiry 47:4, pages 669–693.

In this paper we argue for the existence of covert pied-piping in wh-questions through a previously unnoticed pattern of intervention effects in superiority-obeying English multiple wh-questions. We show that the preference of covert pied-piping is for movement of larger constituents, unlike overt pied-piping. We argue that this discrepancy stems from conflicting requirements of PF and LF: overt pied-piping feeds both LF and PF, but covert pied-piping feeds LF only. The study of covert pied-piping thus reveals the true preference of LF and Core Syntax with regard to pied-piping: larger pied-piping constituents are preferred over smaller ones. This preference can be overridden by certain PF constraints that apply to overt pied-piping.