The converse is to be found in the halakhah and the Talmud where the general principle is that, "a woman cannot be the wife of two " (Kid. 7a and Rashi). For a wife, the term "kiddushin" implies her exclusive dedication to her husband and there can be no "kiddushin" between her and another man while the first "kiddushin" subsists. Any purported marriage to another man is thus formally invalid but, nevertheless, requires a "get" to terminate it. A married man may celebrate a second marriage (and any others) unless he has specifically undertaken to his first wife, e.g., in the "ketubbah", not to do so, or monogamy is the local custom. Thus, Ashkenazic Jews who live in Christian nations accepted a "takkanah" (a rabbinic law not deriving from the Talmud) banning polygamy in c. 1000 CE, while Sephardic Jews who live in Islamic societies have not followed this law.