Sardinian distinguishing features are its articles (derived from Latin IPSE instead of ILLE) and retention of the "hard" sounds of "c" and "g" before "e" and "i". This view is challenged in part by the existence of definite articles continuing forms (e.g. sa mar 'the sea') in some varieties of Catalan, best known as typical of Balearic dialects. According to this view, the next split was between Romanian in the east, and the other languages (the Italo-Western languages) in the west. One of the characteristic features of Romanian is its retention of three of Latin's seven noun cases. The third major split was more evenly divided, between the Italian branch, which comprises many languages spoken in the Italian Peninsula, and the Gallo-Iberian branch. Another proposal However, this is not the only view. Another common classification begins by splitting the Romance languages into two main branches, East and West. The East group includes Romanian, the languages of Corsica and Sardinia, and all languages of Italy south of a line through the cities of Rimini and La Spezia (see La Spezia–Rimini Line). Languages in this group are said to be more conservative, i.e. they retained more features of the original Latin. The West group split into a Gallo-Romance group, which became the Oïl languages (including French), Gallo-Italian, Occitan, Franco-Provençal and Romansh, and an Iberian Romance group which became Spanish and Portuguese. The wave hypothesis Linguists like Jean-Pierre Chambon claim that the various regional languages did not evolve in isolation from their neighbours; on the contrary, they see many changes propagating from the more central regions (Italy and France) towards the periphery (Iberian Peninsula and Romania). These authors see the Romance family as a linkage rather than a tree-like family, and insist that the Wave model is better suited than the Tree model for representing the history of Romance. Degree of separation from Latin In a study by linguist Mario Pei (1949), the degrees of phonological modification of vowels of the Romance languages with respect to the ancestral Latin were found to be as follows Sardinian: 8% Italian: 12% Spanish: 20% Romanian: 23.5% Occitan: 25% Portuguese: 31% French: 44% Some major linguistic features differing among Romance languages Part of the difficulties met in classifying Romance languages is due to the seemingly messy distribution of linguistic innovations across members of the Romance family. While this is a problem for followers of the dominant Tree