Yiddish dialects are variants of the Yiddish language and are divided according to the region in Europe where each developed its distinctiveness. Linguistically, Yiddish is divided in distinct Eastern and Western dialects. While the Western dialects mostly died out in the 19th-century due to Jewish language assimilation into mainstream culture, the Eastern dialects were very vital until most of Eastern European Jewry was wiped out by the Shoah. The Northeastern dialects of Eastern Yiddish were dominant in 20th-century Yiddish culture and academia, but in the 21st-century, since Yiddish is largely dying out everywhere due to language assimilation, the Southern dialects of Yiddish that are preserved by many Hasidic communities, have become the most commonly spoken form of Yiddish. Varieties Yiddish dialects are generally grouped into either Western Yiddish and Eastern Yiddish. Western Yiddish developed from the 9th century in Western-Central Europe, in the region which was called Ashkenaz by Jews, while Eastern Yiddish developed its distinctive features in Eastern Europe after the movement of large numbers of Jews from western to central and eastern Europe. General references to the "Yiddish language" without qualification are normally taken to apply to Eastern Yiddish, unless the subject under consideration is Yiddish literature prior to the 19th century, in which case the focus is more likely to be on Western Yiddish. Western Yiddish While most Jews in the Rhineland who escaped persecution in the 14th century fled to the Polish–Lithuanian Commonwealth, some continued to survive in the countryside of Switzerland, southern Germany and Alsace. They maintained Jewish customs and spoke Western Yiddish. Western Yiddish included three dialects: Northwestern (spoken in Northern Germany and the Netherlands), Midwestern (spoken in central Germany and Luxembourg), Southwestern (spoken in southern Germany, France, and neighboring regions, extending into Northern Italy). These have a number of clearly distinguished regional varieties, such as Judeo-Alsatian, plus many local subvarieties. The language traditionally spoken by the Jews of Alsace is Yédisch-Daïtsch or Judeo-Alsatian, originally a mixture of German, Hebrew and Aramaic idioms and virtually indistinguishable from mainstream Yiddish. From the 12th century onwards, due among other things to the influence of the nearby Rashi school, French linguistic elements aggregated as well, and from the 18th century onwards, some Polish elements due to immigrants blended into Yédisch-Daïtsch too. According to C. J. Hutterer (1969), "In western and central Europe the WY dialects must have died out within a short time during the period of