This article discusses the implications of a new fragmentary manuscript of fifteenth-century polyphony on musicians' biographies, the dating of other manuscripts, and the cultural ties between Venice and the two great Tuscan powers of Florence and Siena. This parchment fragment recovered from a Sienese archival binding represents the earliest surviving concordance between the music of the two Tuscan rivals. It once stood in a Tuscan collection of Holy Week polyphony, including the Lamentations of Jeremiah as set by the Venetian composer Johannes de Quadris; it may have originally been the property of a contado parish church or monastery. The specific musical text of the Siena fragment also can shed new light upon the dating and musical transmission of two contemporary Florentine manuscripts (I-Fn II.I.350 and I-Fd 21), containing music of Brumel, Carpentras and Bernardo Pisano. Finally, the rare Tuscan concordance can begin to clarify some of the earliest known traffic in polyphony between Tuscany and distant Venice.