On 27 February 2014, the British Library acquired the only known complete surviving copy of the Catholicon Anglicum, one of the earliest Middle English-Latin dictionaries, and thereby secured for the nation a key source for the study of English language and lexicography. The manuscript had been in the possession of the Monson family of Burton Hall, Lincolnshire, since the nineteenth century. Edited in 1882 by Sidney H. R. Herrtage – one-time contributor to the Oxford English Dictionary – but not seen publicly since, this fifteenth-century dictionary has so far been accessible solely in modern printed form. This article presents the results of the first detailed codicological analysis of the complete Catholicon manuscript (now Add. MS. 89074). The likely sources of the paper have been identified, as well as the way in which the paper quires were assembled. During binding, parchment sewing guards were inserted around the back and at the centre of each of the sixteen quires. These hitherto unrecorded scraps contain fragments of text that enable the identification of some of their sources: among them, two wills, a memorandum and a charter, all from the fifteenth century. The composition of the Catholicon has traditionally been assigned to north Yorkshire on linguistic and dialect evidence. However, the documents used to make some of the sewing guards yield place-names in the locality of Lincoln (where the manuscript was later owned in 1520), indicating that the binding, at least, may have been made further south.