Baal HaTurim — Rabbi Yaakov ben Asher בעל הטורים — רבי יעקב בן אשר
author of the «Arba'ah Turim», the structure of all Jewish law
Rabbi Yaakov was the son of the Rosh and came with him from Germany to Toledo. He lived in poverty, refused office, and spent his life on one book: the «Arba'ah Turim», the Four Rows, named for the rows of stones on the High Priest's breastplate.
His idea was structural. Rambam had included all law, even the laws of Temple and kingdom that do not apply in exile. Rabbi Yaakov took only what is practised now and divided it into four domains: Orach Chaim — the day, the Sabbath, the festivals; Yoreh Deah — prohibitions, kashrut, mourning, vows; Even HaEzer — marriage and divorce; Choshen Mishpat — property, courts and commerce. Within each topic he presents the views of the Rishonim, the dispute, and the conclusion.
It proved exactly right. Every later code adopted the structure: Rabbi Yosef Karo wrote the Beit Yosef as a commentary on the Tur, then condensed it into the Shulchan Aruch under the same four headings. Halakhah is still cited that way seven centuries later.