Papyrology and Papyri
Papyrology is a compound word formed by two members, “papyrus” (ἡ πάπυρος) and “logos” (ὁ λόγος, «reason», «study», suffix –λογία «discipline»), indicating the scientific study of papyri. The English word «papyrus» is borrowed from Latin, which explains why the correct plural form of the word is «papyri» (think of the second declination), and not «papyruses» (which is recorded in dictionaries, but never used by scholars). A papyrus was manufactured from the Egyptian aquatic papyrus plant. The stem was cut along the length into strips, which were laid crossways in two layers on a flat surface and pressed together in order to form a solid sheet. The solidity of the artifact was guaranteed by the plant’s own natural juices, which served as an efficient glue between the two layers.
http://www.lib.umich.edu/papyrus_making/images/lg_lay.jpg
Sheets were subsequently glued together horizontally, forming a roll. A roll was usually written on its inner surface, where the plant fibers run horizontally and the surface is smoother. This side is conventionally called recto. The outer surface, where papyrus fibers run vertically and the surface is coarser than on the inner side, is called verso.
P.Oxy. 2200. Example of Recto/Verso
Rolls written on both sides are not uncommon. The presence of writing on the verso indicates the intentional reuse of an old exemplar, discarded and then recycled for a different purpose. The papyrus codex, which became popular in the II century, i.e. before the widespread use of parchment, was a format alternative to the roll. A codex was constituted of multiple sheets folded and bound together, like in a modern paperback.
http://www.lib.umich.edu/papyrus-collection/ancient-book-forms