Although in the Middle Ages there is no evidence of dialectalization, perhaps because of the standardizing influence of its official use in the Kingdom of Aragon, since the 16th century the dialects of Valencia and the Balearic Isles, especially, have tended to differentiate from the Central (Barcelona) dialect. Nevertheless, some degree of uniformity is preserved in the literary language, which continued to flourish.
There are two main dialect groups in modern Catalan: Occidental, subdivided into North-West Catalan and Valencian; and Oriental, subdivided into Central dialect, Balearic, Roussillonnais and Alguerese (the dialect spoken in Alghero, Sardinia, where Catalan was introduced in the 14th century). Each of these dialects, except for Alguerese, is in turn divided into subdialects, of which up to eighteen have been identified.
These various dialects differ only in minor respects (details of pronunciation, vocabulary, and verb conjugation) and are easily mutually intelligible. The dialectal differences are not usually reflected in the written language. The Institute of Catalan Studies is responsible for establishing and updating the standard language, which is based on the Barcelona dialect with some admixtures from Valencian. No one spontaneously speaks the standard, but it is used in writing and by the media.
Valencians — except for the intellectual elite — are inclined to consider their speech a separate language. For a number of historical and other reasons, a sector of the Valencian people is deeply distrustful, if not overtly hostile, to Catalans. The ultra-rightist party Unió Valenciana supports this sentiment, and among other things it called for linguistic secession by establishing a separate orthography.