Yiddish took shape among Jewish communities in German-speaking Europe and became the everyday language of Ashkenazi Jewry across a territory that eventually reached from the Rhine to the Dnieper. Its grammar and its commonest words are West Germanic, close enough to German that a German speaker catches much of it, but it is not a dialect of German. Three stocks sit inside one language. The Germanic core supplies the ordinary business of speech. A Hebrew and Aramaic component, called loshn-koydesh, supplies most of the religious, legal and learned vocabulary, and keeps its original spelling rather than being respelled. Centuries of settlement in Poland, Lithuania, Ukraine and Belarus added a Slavic layer of words, diminutives and sentence patterns.
It is written right to left in the Hebrew alphabet. The Germanic and Slavic words are spelled phonetically, with several Hebrew consonant letters pressed into service as vowels, so a reader must know which component a word belongs to before knowing how to read it. The YIVO Institute settled a standard orthography in the interwar years, and it is the one used in scholarship, though Hasidic publishing keeps older conventions.
Dialect divides the language east and west. Western Yiddish, spoken in Germany, Alsace and Bohemia, had largely given way to German by the nineteenth century. The eastern dialects, Lithuanian, Polish and Ukrainian in the usual naming, carried the great majority of speakers, and the standard literary language was built from them. The Holocaust destroyed the communities in which most of those speakers lived. What survived did so through emigration and through the Hasidic communities that rebuilt after the war.