Phonetically, Yiddish is closer to Middle High German than is modern German. Although the vocabulary of Yiddish is basically Germanic, it has been enlarged by borrowings from Hebrew, Aramaic, some Slavic and Romance languages, and English. Written from right to left like Hebrew, Yiddish also uses the Hebrew alphabet with certain modifications.
Yiddish developed somewhere between the 9th and 12th centuries in the Jewish ghettos of southwestern Germany, with a firm foundation in Middle High German dialects, themselves a branch of the Indo-European languages. (Phonetically, Yiddish is closer to Middle High German than is modern German.) Later, when the bulk of European Jewry moved eastward into areas occupied predominantly by Slavic-speaking peoples, Slavic influences were acquired. Yiddish exists in two groups of dialects, one of which is further subdivided. The Western dialect, with few speakers, is centered in German-speaking areas of Western Europe. The more widely distributed Eastern group has a Northeastern branch and a Southern branch. The Northeastern branch includes the Yiddish spoken in the Baltic countries and in the northwestern areas of Russia, and by Jewish immigrants or descendants from those areas. The Southern branch—which has Central and Southeastern subgroups—includes the dialects spoken in Poland, Romania, and Ukraine. Contrary to what many college professors will tell you, Yiddish is alive and well, flourishing particularly in the Orthodox communities of the greater New York area and modern-day Israel.
The vocabulary of the Yiddish spoken in Eastern Europe during recent times comprised about 85 percent German, 10 percent Hebrew, and 5 percent Slavic, with traces of Romanian, French, and other elements. Many English words and phrases entered Yiddish, becoming an integral part of the language as it is spoken in the U.S. and other English-speaking countries. Yiddish is written using the right-to-left letters of the Hebrew alphabet (some of them used differently than for writing Hebrew). Today, Yiddish is spoken by about four million Jews all over the world, especially in Israel, Europe and the U.S, most of which are native to, or have ancestry from, Eastern and Central Europe.