Musical notation appears in liturgical chant manuscripts and also, beginning in the thirteenth century, manuscripts of secular music. For instance, in some manuscripts of the works of Guillaume de Machaut musical notation punctuates narrative and functions as a second layer of illustration. In the later Middle Ages, certain idiosyncratic examples of musical notation are unusually self-referential: these are literally picture songs, graphic scores in which the musical notation outlines the shape of a symbolic object, such as a heart, a harp, or a labyrinth. As scholars of historical sound studies turn to the Middle Ages and early modern period, the visualization of nonmusical sound in the art of the Middle Ages has recently emerged as a new area of research. Alongside the text and music of songs accompanying the ritual, the illuminations depict, and the narrative describes in detail, the instruments and other objects used to produce a nonmusical racket.