Why can speakers of different Basque dialects generally understand one another?

Speakers of different Basque dialects are generally able to understand one another because, despite the existence of several distinct dialects, the linguistic divisions are not significant enough to obscure their common origin.1

While philologist Louis-Lucien Bonaparte identified eight modern dialects, these speech forms share a common underlying sound system and structural foundation. This shared linguistic heritage ensures that the variations between dialects do not prevent mutual intelligibility. Furthermore, the development and widespread acceptance of Euskara Batua ("Unified Basque") has provided a standardized written form that helps bridge regional differences, further supporting communication across the Basque-speaking community.213