In chapter 27 of the last book of his Speculum musice, Jacobus faults an unnamed theorist for misattributing some ars nova doctrine to the ars antiqua; he then excuses the offense by explaining that the oldest ars nova theory might already seem old to current practitioners. This passage and several others suggest that Jacobus was writing at a time when the ars nova was hardly new. And yet the earliest ars nova theory dates from 1319, while the completion of the Speculum musice is often placed in the mid-1320s or ca. 1330. Since the Speculum cites a range of ars nova treatises that in turn cite a repertoire of motets, Jacobus's comments serve as a terminus ante quem for the ars nova writ large. This study reconsiders the date of completion for the last, seventh book of the Speculum musice. It is clear that Jacobus was older than the moderni and finished his treatise as an old man, but he also reveals that he wrote over a long span of time and revised his work repeatedly. His notational proclivities are those of a musician who came of age in a post-Franconian idiom prevalent until ca. 1320, but the latest notational developments he mentions include semiminims, dragmas, and even note shapes otherwise associated with the so-called ars subtilior. In light of this, I suggest that the Speculum musice could have been finished as late as the 1350s by an author in his mid- to late seventies. This redating, in turn, invites broad reconsideration of the transition between ars antiqua and ars nova.

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