The repertory of two-part clausulae associated with the Cathedral of Notre-Dame bears witness to a unique compositional situation in late twelfth- and early thirteenth-century Paris. Developing out of a musical culture shaped by long-established oral practices, the making of this polyphony was, in many ways, reliant upon techniques of memory and ex tempore singing. But in medieval Paris, new attitudes towards the creation and transmission of this liturgical polyphony can be seen to emerge. Opportunities to experiment with musical ideas in the repertory of clausulae in particular, controlling harmonic sounds in measured time, distinguished these pieces as a premier site for the exploration of new musical techniques in polyphonic composition. Co-terminous with these creative efforts, a new interest in the recording of musical practice in writing can also be observed, with thirteenth-century compilers seeking to collect this repertory in fine codices, demonstrating a degree of special notice in the contemporary situation. This dissertation uses the broader context of medieval Paris as a means of introducing surviving clausula collections and examining their content. While previous scholarship has primarily approached the study of clausulae from a chronological standpoint, or an examination of the relationship between the clausula and the motet, less attention has been afforded to the compositional processes and musical techniques that underpin the creation of this repertory. This dissertation attempts to address such a gap. Through an analysis of clausula settings preserved in the three main ‘Notre-Dame sources’ of this repertory (W1, F, and W2), it explores the compositional strategies of medieval singers who fashioned portions of plainchant into rhythmically measured polyphony. Divided into two parts, this study investigates tenor and duplum voices of clausulae in turn, examining their compositional features to illuminate the precise ways that musical materials have been devised, organised, and structured. By placing Parisian manuscript sources of chant at the centre of clausula analysis for the first time, this thesis exposes a spectrum of possibilities available to singers as they set these melodies as clausula tenors. It demonstrates that some tenors reveal a particularly close affinity to practices of singing chant, drawing out and emphasising certain properties of those underlying melodies, while others treat chant melismas in more abstract terms. The range of tenor designs identified here in turn argues for a much more flexible treatment of chant in this polyphonic context than has been acknowledged in scholarship, indicative of singers’ highly creative engagement with their chant heritage. The composition of the upper voices of clausulae is also considered: in a series of case studies, techniques for devising melodic material above a plainchant foundation are categorised and examined. Confronting many of the analytical challenges involved in the study of these melodies, this analysis explores how singers were able to test out melodic ideas in unique, and often particularly sustained ways. A number of recurring compositional techniques that support a view of duplum melodies as deeply interconnected to one another emerge. In addition, a study of these upper voice techniques highlights a more complex relationship between duplum and tenor voices, where duplum composition may be less contingent upon the tenor voice, and in some cases, may play a significant role in the overall compositional form of a clausula setting.