Found:

Pied piping

SYNTAX: 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