The "La Clayette" manuscript (Paris, Bibliothèque nationale de France, nouv. acq. fr. 13521) is large codex of 419 folios, unique among surviving books of thirteenth-century polyphony for combining a substantial fascicle of motets (some 55 in total, over 22 folios) with an otherwise entirely non-musical collection of literary works. Those texts are all in Old French, almost all devotional or didactic in tone. They are vernacular literary materials of precisely the kind read by or to the lay devout, circulated and consumed ever more enthusiastically in the decades after the Fourth Lateran Council of 1215, and used in the cultivation of lay piety. The book has been granted a position in the canon of thirteenth-century polyphonic anthologies only to the extent that its music is considered to have originated separately from its remaining leaves. This dissertation begins with a codicological study of the manuscript, which demonstrates that the music was collected into it for the earliest of three identifiable bindings, probably before the death of Louis IX in 1270. The motets may be considered a cornerstone of the book's didactic project. I argue that the techniques of fascicular construction used to build the codex betray a hopeful uncertainty about where and when written polyphony might be encountered and copied in the vernacular book trade. That uncertainty contrasts with restrictive scholarly conceptions of the motet's social purview; and it is the manuscript's implications for a social history of thirteenth-century polyphony that I set out to explore.

A new paleographical assessment of La Clayette's notation uncovers a broadly consistent set of procedures for notating rhythm, and reveals nuances of rhythmic style not previously identified in published editions of the repertory. Telling paleographical details suggest the book sustained a performance practice in which a single musical reader coached other singers live from the manuscript. This is akin to the practice advocated by La Clayette`s literary works, in which a reader would perform the texts aloud for his audience; though the musical version of the practice incorporated the literature's auditors as singing participants. The only skill required to sing these pieces was the willingness to be taught how. I suggest that the value of singing the motets lay in their ability to produce devotional mental images in ways continuous with the literary texts, and to script a performer's response to them in ways the literary texts could not. Several of the motets engage the memory of ribald vernacular songs, then rewrite that memory devotionally through Latin contrafacture. Thus the pieces offered a devotional training in interpretation itself, one that was contingent upon their musical difficulty, and which cast devotion as a practice adopted by choice and through labor. In later stages of the manuscript's life, its compilers unfolded some of the music's other interpretative possibilities through literary choices that did not fit the volume's first devotional frame. The unruliness of La Clayette's final form betrays changing interpretations of its musical contents over time, and puts pressure on scholarly assumptions about how material texts must anchor the interpretation of music and literature.

Finally, through an analysis of a single motet from the La Clayette manuscript (Par une matinee [807] / Mellis stilla [808] / Alleluia [unidentified]), conducted in dialogue with a paleographical study of each of its fifteen manuscript witnesses, we see how composers could articulate from within motets new ideas about the social domain of music writing in ways that left a lasting legacy to the fourteenth century. I argue that a musicopoetic gambit in the French triplum satirically represents the overheard (but newly composed) song of a shepherdess and her lover as unwritable, and therefore irrational. But its satire is doubly undone, first in that the notational "house style" of La Clayette renders it illegible except through precisely the kinds of oral practices to which it would claim superiority as an written composition; and second, in that the Latin motetus against which the French voice was composed was known far more widely, as a popular sung prayer that did not need writing to endure. While the triplum's style would assert the distinction of the notation in which it was written (in a manner resembling the tendentious, roughly contemporaneous social commentary of Johannes de Grocheio), Mellis stilla suggests that the reach of music writing had limits that did not match the more widespread ability to sing in polyphony. Beyond the written testimony, vernacular polyphony in a style so similar to the motet as sometimes to be indistinguishable from it thrived in ways the triplum's composer probably would not have encouraged, but which our historiographies should now acknowledge.