Context: Catalan dialects are relatively uniform, and are mutually intelligible. They are divided into two blocks, Eastern and Western, differing mostly in pronunciation. The terms "Catalan" and "Valencian" (respectively used in Catalonia and the Valencian Community) are two different varieties of the same language. There are two institutions regulating the two standard varieties, the Institute of Catalan Studies in Catalonia and the Valencian Academy of the Language in Valencia.

Question: What is the basic difference between the dialects?

Answer: pronunciation