No bibliographical information has survived regarding the anonymous treatise Capitulum de vocibus applicatis verbis, the only treatise of the Trecento that provides a description of the musical genres of Italian secular music. The problem of dating the Capitulum is the more challenging for scholars, given the absence of unequivocal points of reference of known date (the presently accepted dating, since Debenedetti’s first description of the source in 1906, is ‘between the years 1313–1332’). The Capitulum and Antonio da Tempo’s Summa artis rithmici vulgaris dictaminis (1332), are connected by the phrase ‘nova sunt pulchritudine decorata’, which appears in the final section of the Capitulum and at the very beginning of da Tempo’s Summa. The original source of this phrase, which does not appear in any other medieval source, is the Constitutio Omnem, a letter written by the Byzantine emperor Justinian I (527–65) on 16 December 533. Antonio da Tempo, who was a judge in Padua, was evidently the first to extract this quotation (with a slight change in the word order) from Justinian’s Constitutio for use in his treatise on poetic forms. For his part, the author of the Capitulum, apparently a simple teacher of grammar and music, took the statement ‘nova sunt’ not directly from Justinian’s Constitutio, but verbatim from da Tempo’s Summa, a source that was closer to his field of professional activity. Therefore, it can be concluded that the Capitulum was compiled not at the beginning of the Trecento but after 1332, the year in which da Tempo completed his Summa.