When the Old French narrative lays were composed in the second half of the twelfth and during the thirteenth century, the authors had access to the theory of classical rhetoric and medieval poetics. This thesis investigates the correspondences between the relevant theoretical concepts and the arrangement of the lays, and reveals in what manner the authors of the lays employed certain techniques and figures in order to achieve the unity of their works. The research is centred on Geoffrey of Vinsauf's Poetria nova and the Documentum de modo et arte dictandi et versificandi, Matthew of Vendôme's Ars versificatoria, and takes into consideration works of the classical origin, namely the Rhetorica ad Herennium and the Ars poetica of Horace. The analysis comprises the lays Aristote, Conseil, Cor, Desiré, Doon, Espervier, Espine, Graelent, Guingamor, Haveloc, Ignaure, Lecheor, Mantel, Melion, Nabaret, Oiselet, Ombre, Trot, Tydorel and Tyolet. The first four chapters discuss the theory on arrangement, assess the unity of the beginning and end with the body of the texts, and examine to what extent the techniques of abbreviation and amplification emphasize or amplify the overall idea. The remaining three chapters demonstrate important structural features typical of the lay narratives which use various rhetorical and poetic methods. Chapter Five analyzes the lay narratives from the perspective of the figures of wordplay, as well as small and large-scale repetition and parallels. Chapter Six examines the recounting of episodes, which coincides with the amplification technique Refining. Chapter Seven exposes the persuasion and reasoning techniques that the characters employ in order to prove or decide something.