He is famous because of his work on the Basque dialects, as he has drawn a new classification and a new map of these dialects—this has been a revolution in a field where few changes were made since the Louis Lucien Bonaparte's works (1863). As the Basque linguist Koldo Mitxelena, he considers that the historical and present characteristics of the Basque language show that this language broke up into dialects in the Middle Ages. Zuazo has identified the places of origin that current Basque dialects derive from; this is the list of those language-innovation centres in chronological order:
^Page on Basque dialects' classification, in the Media Archive of the Basque Language in Navarre (a project of Euskarabidea, the Basque Language Institute of the Government of Navarre). Retrieved on 2010-08-22.