The final volumes of a project begun in 1921 by the Board of Celtic Studies of the University of Wales to produce the first standard historical Welsh dictionary, these dictionaries present in alphabetical order the vocabulary of the Welsh language from the remnants of Old Welsh, through the abundant literature of the Medieval and Modern periods, to the explosion of vocabulary arising from the ever-increasing use of the language in all fields of 20th-century life. These references are the first standard historical Welsh dictionaries. The vocabulary is defined in Welsh, but English equivalents are also given. Detailed attention is given to variant forms, common collocations, and etymology.