Medieval manuscripts are described by scholars by the library in which they are located and the shelfmark (a term similar to the United States "call number") of the manuscript within the library.
Sir Gawain and the Green Knight is preserved in a single manuscript, MS. Cotton Nero A. x, which also contain three religious texts, Pearl, Purity, and Patience.
The term Cotton refers to the Cotton Library, a collection started by Robert Cotton, a late 16th/early 17th century collector, and gradually added to by his family until it was bequeathed to the British Nation in 1701 and became one of the most important manuscript collections in the newly formed British Museum (now the British Library) in 1753. The area in which it was stored in the old British Library consists of a row of shelves located under busts of the 12 Caesars, and thus the shelf marks consist of the name of a Roman emperor, followed by an indicator of where in the area under that emperor's bust the manuscript was located. Thus MS. Cotton Nero A. x means a manuscript that is part of the Cotton collection in the British Library that was stored under a bust of Nero in the Old British Museum (before the recent move to St. Pancras).