In the early sixteenth century choirbooks gradually passed out of use in favor of partbooks. (At least for a time some choirs seem to have preferred one type of manuscript, some the other, for in 1524 all the polyphonic music at Magdalen College, Oxford was contained in choirbooks, nine of which had been bought between 1518 and 1524, while in 1529 King's College, Cambridge relied almost entirely on partbooks). Partbooks with small paper leaves were undoubtedly cheaper to produce than large elaborately bound choirbooks of parchment. They were also easier to handle, and probably more convenient for a large number of singers to read from. The choir would no longer gather round a lectern, but would presumably sing from the choir-stalls, except in votive antiphons performed before images, where, unless singing from memory was the practice, each book must have been held by one or more singers for all of those on that part to see.
The number of partbooks surviving is quite considerable, but complete sets are very few. These few include the important Forrest-Heyther and Gyffard sets and the source of Ludford's Lady Masses, British Museum Royal Appendix MSS. 45-48. The Peterhouse and Christ Church sets both lack their tenor books; reconstruction is easy where proper plainsongs in equal notes are involved, but at other times it is an awkward task.
The Forrest-Heyther partbooks (Oxford, Bodleian Library MSS. Mus. Sch. e. 376-81)8 are a major source of large-scale Masses. In 1530 they belonged to a petty-canon of Cardinal College, Oxford, William Forrest, for an inscription in MS. 378 reads: 'William Forrest hunc librum jurae possidet cum quinque alijs eidem pertinentibus, 1530'. Taverner, first choirmaster at the new College from 1526 to 1530, had almost certainly supervised their compilation--or, to be precise, the compilation of the first layer of eleven Masses. His six-part Gloria tibi Trinitas heads the collection, very impressively, since nine of the ten Masses following are for five voices, and fittingly, because it is built upon an antiphon of the Trinity, to whom the College was dedicated. The other Masses of the first layer include four by Fayrfax, and John Merbecke's Per arma justitiae. Possibly on Taverner's departure in 1530 it was decided that less ambitious music should be used, and Forrest rescued the six partbooks from destruction: exactly why or how else he should be able to lay personal claim to them at this time is uncertain, for one would expect them to be the property of the College, not of an individual.
The second layer, dating from the mid sixteenth century, is probably in Forrest's own hand (except for the last part of the sixth ('sextus') book, which was copied by John Baldwin very much later). It begins with four items perhaps included in the original plan of the manuscript (Taverner's Corona spinea and O Michael, Ashewell's Ave Maria and Aston's Videte manus meas); but the last three Masses (Tye's Euge bone, Sheppard's Cantate and Allwood'sPraise Him Praiseworthy) are clearly much later works. When Baldwin completed the sextus book, and possibly even when the rest of the second layer was added, the books had become (as some later manuscripts were from their inception) of primarily musical and antiquarian interest, rather than part of a living liturgical tradition.
The first layer of the Forrest-Heyther partbooks (from the late 1520s) is probably the earliest source for Taverner and Aston. In addition to one Mass by each of these composers, it contains four Masses by Fayrfax and one each by Thomas Ashewell, John Merbecke, John Norman and William Rasor. The second (mid-sixteenth century) layer has a second Mass by Ashewell among further works by Taverner and Aston and music by younger composers, Sheppard, Tye and Richard Allwood).