I know relatively (SWIDT) little about the development of relative pronouns in OE. From what I understand, the practice of using interrogatives as relative pronouns is a Modern English practice that wasn't present in OE; OE instead used demonstratives. Assuming that I am right about this:
- What prompted this development? After all, it's not as though OE was lacking in relative pronouns.
- Was it some kind of regional effect (since it appears that other SAE languages have undergone something of a similar development)?
- Could OE relativise possessives? (I saw the man whose boat I borrowed?) If it couldn't, I suppose I could understand how wh-words might be roped into service on that front.