The manuscript Munich, Bayerische Staatsbibliothek, Clm 14274 (now also ‘Tresorhandschrift l’) is a large collection of mensural polyphonic music, mostly composed in the first half of the fifteenth century, although a few pieces date back to the late fourteenth. Apart from its importance as a musical source (more than half the compositions it contains are unknown from other sources), Clm 14274 is the geographically northernmost representative of the small group of manuscripts from northern Italy and southern Germany which contain the core of the surviving repertory of early-fifteenth-century polyphony, and, as such, provides potentially vital documentary material for the study of this repertory.