What was J.R.R. Tolkien's academic field at Oxford before his 1945 chair?

Before his 1945 chair, J.R.R. Tolkien's academic field at Oxford was Anglo-Saxon.

Tolkien became Rawlinson and Bosworth Professor of Anglo-Saxon at Oxford University in 1925. The field, now commonly described through Old English studies, concerned the language and literature of early medieval England. His scholarship included work on texts, vocabulary, philology, and the history of English.

In 1945, Tolkien moved to the Merton Professorship of English Language and Literature, a position he held until his retirement in 1959. His professional knowledge of historical languages strongly informed his fiction, including the relationship between invented languages, peoples, and histories.

The term Anglo-Saxon in this question refers to the academic chair and period designation used at Oxford. It does not mean Tolkien studied only one text or that his fiction was simply a retelling of Anglo-Saxon literature.