The internal classification of the Romance languages is a complex and sometimes controversial topic which may not have one single answer. Several classifications have been proposed, based on different criteria. Attempts at classifying Romance languages Difficulties of classification The comparative method used by linguists to build family language trees is based on the assumption that the member languages evolved from a single proto-language by a sequence of binary splits, separated by many centuries. With that hypothesis, and the glottochronological assumption that the degree of linguistic change is roughly proportional to elapsed time, the sequence of splits can be deduced by measuring the differences between the members. However, the history of Romance languages, as we know it, makes the first assumption rather problematic. While the Roman Empire lasted, its educational policies and the natural mobility of its soldiers and administrative officials probably ensured some degree of linguistic homogeneity throughout its territory. Even if there were differences between the Vulgar Latin spoken in different regions, it is doubtful whether there were any sharp boundaries between the various dialects. On the other hand, after the Empire's collapse, the population of Latin speakers was separated—almost instantaneously, by the standards of historical linguistics—into a large number of politically independent states and feudal domains whose populations were largely bound to the land. These units then interacted, merged and split in various ways over the next fifteen centuries, possibly influenced by languages external to the family (as in the so-called Balkan language area). In summary, the history of Latin and Romance-speaking peoples can hardly be described by a binary branching pattern; therefore, one may argue that any attempt to fit the Romance languages into a tree structure is inherently flawed. In this regard, the genealogical structure of languages forms a typical linkage. On the other hand, the tree structure may be meaningfully applied to any subfamilies of Romance whose members did diverge from a common ancestor by binary splits. That may be the case, for example, of the dialects of Spanish and Portuguese spoken in different countries, or the regional variants of spoken standard Italian (but not the so-called "Italian dialects", which are distinct languages that evolved directly from Vulgar Latin). The standard proposal Nevertheless, by applying the comparative method, some linguists have concluded that the earliest split in the Romance family tree was between Sardinian and the remaining group, called Continental Romance. Among the many peculiar