Rabbeinu Asher (the Rosh) in Spain and his son's Arba'ah Turim

Rabbeinu Asher ben Yechiel was the favourite student of the Maharam of Rothenburg. When the emperor imprisoned the Maharam and demanded an enormous ransom, the Maharam forbade the communities to pay, lest the kidnapping of rabbis be encouraged, and died in captivity. The Rosh, left as head of German Jewry, understood that staying was dangerous. Around 5063 he fled, and in 5065 reached Toledo, where the community made him chief rabbi.

Two traditions that had run apart for centuries met in one man. The Rosh brought the Tosafist method of sharp analysis; the Spaniards gave him the disciplined structure of the Rif and Maimonides. His Piskei HaRosh is a code arranged by tractate in which Ashkenazi views stand for the first time systematically beside Sephardi ones. Rabbi Yosef Karo would later make him one of the three pillars by whose majority the law is fixed.

His son Rabbi Yaakov ben Asher completed the work. Around 5100 he finished the Arba'ah Turim, the 'Four Columns': Orach Chaim (daily life, prayer, Shabbat, festivals), Yoreh De'ah (kashrut, mourning, vows), Even HaEzer (marriage and divorce), Choshen Mishpat (property and courts). Unlike Maimonides he did not conceal disagreement, citing the Rif, Maimonides, the Rosh and others before giving his conclusion, and he kept only what applies in exile.

That fourfold division is the frame Karo took for the Shulchan Aruch.