The gutter, folds, and edge divide the sheet into rectangular panels. Some panels form logical pages of their own, but some logical pages span two panels.
Some panels bear the folio number in their upper right corners.
This shows the 9 rosette diagram. The crease separating e from f is disintegrating. If you fold the horizontal crease the 9 rosette diagram is hidden from view, and what you see is describable by the regular notation.