This chapter traces a single plainchant melody across three different polyphonic genres, documenting and synthesising its behaviour in more than thirty individual compositions. Outlining key terms and musical procedures in thirteenth-century polyphony, it demonstrates precisely how a plainchant melody is approached and treated in a polyphonic organum, how sections of these organa become independent as clausulae, and how these clausulae then relate to self-contained motets with Latin and French texts. Serving as an accessible introduction to thirteenth-century musical practices and chronologies, the chapter simultaneously problematises their conventional historiographical narratives. Not only do the three genres in question relate to one another here in unexpected and complex ways, but the plainchant foundation itself is subject to musical manipulations that are highly unusual. The chapter challenges current understandings of the compositional freedom with which liturgical melodies might be treated in sacred contexts, conversely demonstrating a surprising potential for adherence to liturgical orthodoxy in vernacular ones.