Which medieval poem did J.R.R. Tolkien co-edit with E.V. Gordon in 1925?

J.R.R. Tolkien co-edited Sir Gawain and the Green Knight with E.V. Gordon in 1925.

Their edition was published by the Clarendon Press and became an important scholarly treatment of the Middle English poem. Tolkien contributed his expertise in language and literature, while Gordon was his colleague and collaborator. The work presents a late fourteenth-century Arthurian romance in which Sir Gawain accepts a supernatural challenge from the Green Knight.

Tolkien’s interest in the poem went beyond editing its text. Its alliterative verse, moral testing, feasting, landscape, and blend of Christian and older heroic traditions were close to the kinds of medieval materials that informed his imagination. He later gave a well-known lecture on the poem, published posthumously as Sir Gawain and the Green Knight, Pearl and Sir Orfeo.

The poem is often confused with Pearl because both survive in the same manuscript, the Cotton Nero A.x manuscript. They are separate works, although Tolkien studied and edited both traditions.