X = Vat. gr. 191 (Vatican), a paper codex containing a large corpus of mathematical and scientific writings, copied by numerous hands and assembled about 1296.
The text of the Geography was originally copied by three hands, and some missing pages in the beginning have been replace by a fourth; there are no maps. For some unknown reason the second and third scribes omitted all the numerical coordinates in the geographical catalogue from Book 5.13 on. In spite of this serious defect, X is a manuscript of the greatest importance for the text of the Geography, because it is the only copy that is uninfluenced by the Byzantine revision.
(Quoted directly from [2].)