A number of manuscripts from Southern Italy, almost all of them written in ‘Beneventan’ script and neumes, contain chants that are not the usual Gregorian assignments. These unusual chants have come generally to be known as Old Beneventan; in the manuscripts, however, a number of them are designated “Ambrosian”. That these chants are in any real sense Ambrosian has never been taken very seriously, even though there is very good evidence for doing so.

The melody for the verses beginning “Trinitas, Unitas, Deitas” has so far been published only from three French sources of the 13th century. Amedee Gastoue gave the melody in the course of his essay ‘Les anciens chants liturgiques des Églises d'Apt et du Comtat’; his transcription was based on the manuscript Apt 6 (No. du fonds 1), a mass-antiphoner of the 12th century used at the church of St.Pierre in Apt, with additions in various hands on the unfoliated leaves after folio CXII at the end of the manuscript, which is where we find Trinitas, Unitas, Deitas, between a troped Kyrie and a Christmas song. Henri Villetard (1907) and Wulf Arlt (1970) published the melody as part of their respective editions and commentaries for the New Year's offices of Sens and Beauvais, from the manuscripts Sens 46 and London, B.L., Egerton 2615, that is, from two sources of the early 13th century whose contents are related, and which may be seen as recensions of a 12th-century exemplar no longer extant.

The Lambeth and Caius Choirbooks (London, Lambeth Palace Library, ms.1; Cambridge, Gonville and Caius College, ms.667/760 [since 1947 deposited in the University Library, Cambridge]) are two of the most important sources of music by composers of the generations of Fayrfax and Ludford. Their importance is heightened by the fact that, of the scores of such choirbooks that must once have existed in England, only one other, the famous Eton Choirbook (Eton College, ms.178.), survives in anything remotely like a complete state.