Which English reference work did J.R.R. Tolkien help compile after World War I?

J.R.R. Tolkien helped compile the Oxford English Dictionary after World War I.

He worked on the dictionary from January 1919 until 1920, initially as a temporary assistant. His responsibilities included researching the history and usage of words, especially terms beginning with letters in the range from waggle to warlock. This work gave him practical experience in tracing how English vocabulary changed across centuries.

The OED was a major scholarly project that documented words through dated quotations and historical evidence. Tolkien’s involvement connected his professional philology with the lexicographic methods that later supported his invented languages. The job is sometimes described as Tolkien writing the dictionary, but he was one contributor among many researchers and editors. His later academic career at Leeds and Oxford continued the study of language history and medieval literature.