The purpose of this study is to conduct a close and careful study of the metre of Sir Gawain and the Green Knight, and thereby to describe the metrical principles that underlie the structure of the unrhymed long line, especially, that of the a-verse, and to demonstrate the stylistic possibilities that individual poets could exploit on the basis of these principles. In the introduction, I re-examine the three-stave half-line theory and point out the inconsistencies and unnecessary complexities that this theory entails, and argue for the regular two-stave verse and the potential disjunction between alliteration and stress. Chapter I examines the lines with non-aa/ax patterns found in Sir Gawain, and considers whether the non-aa/ax alliterative patterns in this romance should be treated as `irregular' and thus be assumed to require emendation. Chapter II deals with the so-called `extended' verses, and how stress and alliteration function in such half-lines; Chapter III investigates combinations of various syntactic units, mainly those of adjective + noun and verb + adverb, and presents general metrical `rules' which appear to govern the `extended' and non-'extended' a-verse; Chapter IV is aimed at the demonstration of these rules by examining the metrical function in the long line of doublet forms, such as to/for to + infinitive and on/ vpon folde. Chapter V presents a comparative study between the metre of Sir Gawain and that of Cleanness and Patience, the other alliterative poems found in the same manuscript, and three other alliterative poems, namely, The Destruction of Troy, The Wars of Alexander, and St Erkenwald. Chapter VI explores how the alliterative metre can be exploited for stylistic purposes. My conclusions smmarisetsh e metrical rules that have emerged from this study.