Which medieval poem did J.R.R. Tolkien and E.V. Gordon publish a major scholarly edition of in 1925?

J.R.R. Tolkien and E.V. Gordon published a major scholarly edition of Sir Gawain and the Green Knight in 1925.

The poem is a Middle English Arthurian romance dating from the late 14th century. It follows Sir Gawain, a knight of King Arthur's court, as he accepts the Green Knight's beheading challenge and later travels to meet the mysterious challenger at the Green Chapel.

Tolkien and Gordon's edition helped establish the poem as an important subject for modern literary study. Tolkien contributed expertise in medieval languages and literature, while Gordon was his colleague at the University of Leeds. Tolkien later translated the poem into modern English; that translation was published posthumously in 1975.

The work is often grouped with Pearl and Sir Orfeo because they survive in the same manuscript, Cotton Nero A.x. It is not the same poem as Beowulf, which Tolkien also studied extensively.