Around the year 1000, butterfly binding was developed. Woodblock prints allowed two mirror images to be easily replicated on a single sheet. Thus two pages were printed on a sheet, which was then folded inwards. The sheets were then pasted together at the fold to make a codex with alternate openings of printed and blank pairs of pages. In the 14th century the folding was reversed outwards to give continuous printed pages, each backed by a blank hidden page. Later the sewn bindings were preferred rather than pasted bindings. Only relatively small volumes ("juan 卷") were bound up, and several of these would be enclosed in a cover called a "tao", with wooden boards at front and back, and loops and pegs to close up the book when not in use. For example, one complete Tripitaka had over 6,400 "juan" in 595 "tao".