The Roman emperor whose bust gave Cotton Vitellius A XV its name was Vitellius.
The designation belongs to the library system of Sir Robert Cotton, a major English manuscript collector. Cotton organized his books by placing them in bookcases identified by busts of historical or mythological figures. A manuscript kept in the case marked by a bust of the emperor Vitellius received “Vitellius” in its shelfmark.
“ A XV” completes the classification: the manuscript was on shelf A and was the fifteenth volume in that section. The full designation is therefore Cotton MS Vitellius A XV. The Nowell Codex is one of two manuscripts later bound together within that volume, alongside the Southwick or Southwell Codex.
The name does not mean that the manuscript was written during Vitellius’s reign or that the emperor had any connection with Beowulf. Vitellius ruled briefly in AD 69, during the Roman civil war known as the Year of the Four Emperors. In this case, his bust served simply as a library locator.