Relative clause constructions in the world's languages are rather diverse in terms of their general structure, marking strategies and their requirements/restrictions on the verb and head types. At the same time, however, there are certain ``types'' and patterns that are found again and again, for example the use of demonstratives or interrogatives as relative markers, correlations between word order and the position of the relative clause with regard to its head, use of genitive subjects with reduced relative clauses, and much more.
My thesis examines the various diachronic processes that lead to these patterns and variation. Specifically I am addressing the following questions:
- What sort of vocabulary items take on the role of relative markers and what other functions these items can be extended to (for example focus-marking, general complementizers, and adjective markers, among others).
- What other changes can affect relative clause constructions -- changes in mood/tense restrictions on the verb, agreement patterns, clause boundary shifts, etc.
- Whether the generalisations found can be described and accounted for within frameworks such as ``grammaticalisation theory'' and recent formal syntactic theories.

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