The Biffoli-Sostegni manuscript, MS 27766 of the Bibliothèque du Conservatoire royal de Bruxelles, is the only volume of sixteenth-century polyphony with a secure provenance in a female convent. Its extraordinary survival is made all the more important by its origin at the Florentine convent of San Matteo in Arcetri, the convent in which Suor Maria Celeste Galilei, daughter of Galileo Galilei, spent the last two-thirds of her life. This Element uses archival sources related to San Matteo to create a historical context for the manuscript's music and the lives of the nuns for whom it was written. Analysis of the music is accompanied by both notated and audiovisual musical examples, performed by the UK all-female early music ensemble, Musica Secreta.