The physical object is a screenfold: strips of tanned deerskin glued end to end, folded concertina-fashion into 39 leaves of about 27 centimetres square, and surfaced with a thin white gesso that takes paint the way plaster does. Every leaf but the two outermost is painted on both sides, giving 76 painted pages, and the whole opens to something over ten metres. Leather covers protected the ends. Nothing about it is European: the format, the materials and the pigments are those of pre-conquest central Mexico, and the book was made to be opened at the panel needed rather than turned page by page.
Its organising subject is the tonalpohualli, the 260-day divinatory count formed by running thirteen numbers against twenty day signs. That count governed naming, marriage, war, planting and sacrifice, and a book like this was a working instrument for the specialist who read it rather than a devotional text. The pages are laid out as almanacs: the count divided into fives, into twenties, into the nine Lords of the Night, each division attached to particular deities, offerings, birds, trees and directions, with the fortune of the period signalled by what is shown rather than by anything written.
There is no phonetic writing in it at all. The Borgia Group manuscripts encode meaning entirely in image and convention, which is why they can be read by scholars working across languages and why the linguistic identity of the painters is still argued over. Its origin is disputed on the same grounds: the Puebla-Tlaxcala region and western Oaxaca are the usual proposals, and the painting belongs to what is called the Mixteca-Puebla style, so calling the book flatly Aztec overstates what is known. The reading order generally runs right to left, which matters because the long sequence in the middle of the book breaks that rule.