This study is an attempt to bridge the gap between the formalism and theoretical constructs introduced by generative grammar, whose ultimate goal is to provide not only a description but also an explanation to linguistic phenomena, and historical linguistics, which studies the evolution of language over time. This main objective is met by providing a formal account of the changes that hwæðer ‘whether’ undergoes throughout the Old English period. A classification of its diverse functions was possible through the identification and analysis of all instances found in the diachronic part of the Helsinki Corpus. Both quantitative and qualitative methods were employed in an analysis which proves to be indispensable for understanding how language works at the abstract level from a historical point of view. The project is part of a growing body of research on language change which attempts to describe and explain the evolution of certain words as these change in form and function. The book therefore presents valuable information for scholars and students alike interested in morpho-syntactic historical changes.