The "regular" VMS folios, like those in Herbal, are listed as measuring ~160 by ~230 mm, meaning that each "regular" bifolio is about ~320 x ~230 mm.
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Login to view., that allow a sheet of one size to be cut or folded into two halves with the same aspect ratio. Specifically, each bifolio of the VMS is close to the ISO C4 size
324 by 229 mm, which is the size of an envelope suitable for a document in the common A4 paper format.
Quires 14 and 15, 17, and 19 consist of larger "non-regular" sheets. (Quires 16 and 18 are assumed to have been lost after the folios were numbered.)
Quire 14, the Rosettes fold-out, has a single sheet with two rows of three flaps. Four of the flaps are fairly rectangular with about the same width as a regular folio. (The top left flap, however, seems to have a curved top edge and to be a bit shorter vertically.) The two rightmost flaps have an irregular right edge and vary in width from ~130 to ~145 mm.
The inner "bifolio" of quire 15, f88+f89 has four flaps: two fairly rectangular ones with normal dimensions, then a slightly narrower one, ~145 mm wide, and a very narrow one with very irregular right edge, ~110 mm wide. (The latter is also ~5 mm shorter vertically.)
Another "bifolio" with four flaps is f99+f102, the outer "bifolio" of quire 19. Like the one above, it has two "regular" flaps, then one that is a bit narrower has only three flaps. The first two have "regular" shape and size; the third one is still fairly rectangular but a bit narrower, ~130 mm; and the fourth one has an irregular edge and is very narrow, from ~30 mm at the bottom to ~50 mm at the top. (The third flap is missing a triangular piece at the bottom corner adjacent to flap 2.)
It s proposed that the Author obtained the vellum of quires 14-19 as a set of six off-cuts whose height was twice that of a regular folio, ~460 mm, with two straight horizontal edges and one straight vertical edge, whose width varied irregularly between ~480 and ~640 mm, like the "B" and "C" red outlines below:
It is conjectured that the vellum maker or the merchant intended to cut the prepared skins into sheets of 640 by 460 mm (close to the ISO C2 paper), as per the blue outline above. A sheet of that size would have been a bifolio of a book twice as tall and wide as the VMS. Let's say that it would sell for 10 pazoozas.
However he would get many off-cuts like the red outlines above. Presumably he would further cut any one with an "A" outline along the dashed vertical line to produce a "C3" sheet, suitable as a bifolio of a book 40% wider and taller as the VMS. Let's say that it would sell for 4 ₽.
On the other hand, an off-cut with "B" or "C" outline would be offered "as is" to any consumer who could use them. Its market price would be somewhere between 4 and 10 ₽ -- say, 6 ₽.
So the Author would have acquired a bunch of those "B" or "C" off-cuts. He used a whole "B" one, trimming the irregular edge, for the Rosettes diagram. He cut the others horizontally, using the "B" ones for "bifolios" with three "regular" flaps, like f87+f90, and the "C" ones for "bifolios" with three-and-a-half flaps, like f88+f89.
Makes sense?
All the best, --stolfi