Pied piping

Pied piping is the phenomenon that when a wh-phrase is moved, it can optionally 'drag along' a larger NP or PP in which it is contained.

Example
next to (1a), (1b) and (1c) are also possible.

(i) a This is the book [NP which] I have designed [NP the covers [PP of t]] b This is the book [PP of which] I have designed [NP the covers t]     c  This is the book [NP the covers of which] I have designed t

In some cases, Pied Piping is obligatory, due to the Left Branch Condition.

Links
Utrecht Lexicon of Linguistics