This manuscript consists of the Contratenor only of what was originally a set of six partbooks. Had the collection survived intact, it would undoubtedly have ranked as one of the most important of all secular sources dating from the Elizabethan period, for it comprises a representative cross-section of most musical genres—English anthems, Latin motets, consort songs and instrumental pieces—and its 131 items include many unica. But the sheer size of the anthology is not the only reason why we should lament its tragically imperfect state. Mus. Sch. E. 423 is an important and authoritative source for the vocal music of William Byrd, as Philip Brett, Alan Brown and other scholars have shown. Writing more generally about the instrumental music it contains, Warwick Edwards has stated that its ‘authority as a consort source derives from its exclusion of all but a few faults against other sources’. The manuscript's special relevance to Byrd, then, not the mention the high quality of its musical and verbal texts, makes the loss of its companion books all the more regrettable.