Modern Albanian is derived from the ancient Illyrian and Thracian, but many outside influences are evident. Additions and modifications, beginning in the pre-Christian era, were made as a result of foreign contacts. Most important of these were the Latin and Italian influences during the centuries of Roman domination and trade with the Venetian merchants and, later, in the nineteenth and twentieth centuries. Contributions also were made by the Greeks, Turks, and Slavs. The first written documents in Albanian did not appear until the fifteenth century; therefore it is difficult to trace the development of the language during the earlier period. The repressive policies of the Ottoman rulers over a period of 450 years, beginning in the fifteenth century, further retarded language development. Written Albanian was forbidden, and only the Turkish or Greek languages could be used in schools. Emigré Albanians, particularly those in Italy after 1848, helped keep the written language alive. Until the nineteenth century continuity of the language in Turkish-dominated areas was provided largely by verbal communication, including ballads and folk tales (see ch. 7, Communications and Cultural Development). By the early twentieth century more than a dozen different alphabets had developed. Some were predominantly either Latin, Greek, or Turko-Arabic. Many were a mixture of several forms. It was not until 1908 that a standardized orthography was adopted. The Latin-based alphabet of thirty-six letters, approved at that time by a linguistic congress at Monastir, was made official by a government directive in 1924 and continued in use in 1970. Letters are written as they are pronounced. There have been variations in the spelling of many words because of dialectical differences, and they still persist despite the government's efforts to develop a uniform language. A dictionary was published by the Institute of Sciences in Tirana in 1954, and it indicated that the spelling of some words varied. During the 1960s the Linguistics and History Institute, which was part of the State University of Tirana, carried on studies relating to language origins and morphology, but no lexicon was known to have appeared as of early 1970 to standardize spelling or supersede the 1954 dictionary. The two principal Albanian dialects are Geg, spoken by about two-thirds of the people, including those in the Kosovo region of Yugoslavia, and Tosk, by the remaining third. There are subvarieties of both dialects. Despite the considerable variations that developed in the many isolated communities, Albanians are able to communicate easily with each other.