In Europe the fold-out seems to appear (though it may be a re-appearance) fairly suddenly, around the late thirteenth century, and always in the same contexts: farmer’s calendar, physician’s or preacher’s vade mecum, and so on. The example shown (above) is late thirteenth century, a merchant’s calendar made in Paris. Its sections each again appear as the page of an ordinary Latin book, nicely ruled-out, and now with folds arranged as if they were letters i.e with the sides turned over towards the centre. Not like the Voynich manuscript’s foldouts, though they too are on vellum.
Here we have to recall that Wilfrid Voynich believed the manuscript Roger Bacon’s, and Bacon a member of the Franciscan preaching order of Friars Minor. The overall look of our manuscript also bears comparison with Franciscan handbooks, as we have seen except that our remaining examples are on parchment, not rough vellum. We don’t hear of Bacon’s preaching, but of his scholarship and teaching. Nevertheless, and though I have no examples to show you of Franciscan works containing fold-outs, it is possible that our book might be a travelling friar’s.
When the term “vademecum’ is applied to more modern religious works, it usually describes a handbook in which there are prepared sermons, the ecclesiastical calendar and other useful bits.
It is also a term used to describe a ninth-century manuscript of useful odds-and-ends. The work of Walafrid [Wilifrid] Strabo, it is St. Gallen Stiftsbibliothek Cod. Sang. 878. One page of it is online here.
This category is made by analogy. The great pictorial compendium made by Cresques Abraham (also: Abraham Cresques) in 1375 incorporates a great number of charts and tables including those intended for calculating the position of the tides and moon. As the ‘Atlas Catala’ it was owned by the court of France, but if one envisages it smaller, the contents would perfectly suit a small manuscript which had fold-outs and indeed some of those included are known from earlier as from later manuscripts. Like the navigator, the geographer and chart-maker constantly made use of diagrams and other figures, while the ‘mappamundi’ sort of map filled its landscapes with images of regional animals, peoples and plants.
Apart from its world-map (or possibly even with it) the “Atlas” is in fact the rendering in pictures and diagrams of the content from an Islamic Almanac or of the type of work produced and used in North Africa following the new and cross-disciplinary style of al-Idrisi’s “Amusement..”.
It is possible, then, that our manuscript could be a ready-reference for the chart-maker, but we have no examples of any similar handbooks made for that purpose – as far as I’m aware.
GENEALOGIST or HERALD?
I do not see our manuscript’s fold-outs as likely to refer to genealogical trees or formal heraldry, though allusion to these matters in the written part of the text is conceivable. As illustration of such matter, which became a very common reason for including fold-out charts, here is a seventeenth century example: an edition of Savanorola’s biography, first published in 1537 ~ here.
1. I should like to have seen Schoenberg 2655, listed as a “Franciscan vademecum. 1463. Northern Italy. On vellum. 122 x 86 cm. It appears not to be digitised yet. There is also the intriguing suggestion that the “Codex Cardona” might have been made by Franciscans, as the Florentine codex was. On the Codex Cardona see, e.g. here. It cannot possibly be the Voynich manuscript, which had been in the Beinecke Library at Yale for fifteen years when the Cardona turned up at Sotheby’s in 1982.
2. Pico della Mirandola, Giovanni Francesco. Vita R. P. Fr. Hieronymi Savonarolae ferrariensis, ord. praedicatorum. Paris: Sumptibus Ludovici Billaine, 1674. 12mo (15 cm, 5.9″). Vol. I of II. Frontis.,  ff., 385 [i.e., 375],  pp. Plates.