On this account it seems more than likely that influential Polish Jews cooperated with the leading Lithuanian communities in securing a special charter from Vytautas (Witold).
Under the charter, the Lithuanian Jews formed a class of freemen subject in all criminal cases directly to the jurisdiction of the grand duke and his official representatives, and in petty suits to the jurisdiction of local officials on an equal footing with the lesser nobles (szlachta), boyars, and other free citizens.
The traditional language of the vast majority of Jews of Lithuania, Yiddish, is based largely upon the Medieval German spoken by the western Germanic Jewish immigrants.
The peculiar conditions that prevailed in Lithuania compelled the first Jewish settlers to adopt a different mode of life from that followed by their western co-religionists.
In matters of religion the Jews were given extensive autonomy.
Under these equitable laws the Jews of Lithuania reached a degree of prosperity unknown to their Polish and German co-religionists at that time.
After the death of Casimir III (1370), the condition of the Polish Jews changed for the worse.