He is especially interested in place-names that demonstrate the influence of both Welsh and English on each other and he is author of many books that trace the origins and meanings of place-names.
Ymddiddora’n arbennig mewn enwau sy’n dangos dylanwad y Gymraeg a’r Saesneg ar ei gilydd ac mae’n awdur ar sawl cyfrol sy’n olrhain tarddiad ac ystyr enwau lleoedd.