Rabbeinu Tam (c.1100 - c.1171) (רבינו תם) (real name: Rabbeinu Yaakov, or Jacob in English language) was one of the Baalei Tosafos whose commentary appears in every edition of Talmud opposite the commentary of Rashi. He is also well-known for his halakha opinion on the proper order of the scrolls placed in the tefillin, which diverges from that of Rashi. Today both "Rashi tefillin" and "Rabbeinu Tam tefillin" are produced, and some Jews wear both in order to satisfy both halakhic opinions.
Rabbeinu Tam was born in the French country village of Remeruque (now Ramerupt, France), to Rabbeinu Meir ben Shmuel and his wife, Yocheved, the daughter of Rashi. Legend has it that when Rashi was holding his infant grandson, the baby touched the tefillin that were on Rashi's head. Rashi predicted that this grandson would later disagree with him about the order of the scripts that are put in the tefillin.
Halakhic differences of opinion
Rabbeinu Tam grew up to become the foremost halakhic authority of his generation and one of the greatest leaders of his time. He was one of the most famous of the Tosafists. He acquired his name, Tam, because his Bible namesake, Jacob, is described as "tam", implying straightforward and truthful.
While the Tosafists' rulings tended to side with Ashkenazi Jews tradition (and thus with Rashi's halakhic opinions), Rabbeinu Tam's rulings often disagreed with that of his grandfather. The most well-known difference of opinion is Rabbeinu's stance on the ordering of the scrolls placed in tefillin. Of the four parchments placed inside the tefillin shel rosh and tefillin shel yad (tefillin worn on the head and arm, respectively), Rashi ruled that the passage of the Shema Yisrael ("Hear O Israel") should precede that of, "And it shall come to pass, if you hearken...". Rabbeinu Tam held that the ordering of these parchments should be reversed.
While most Ashkenazi Jews follow Rashi's opinion, some Jews (particularly Hasidim), first wear Rashi tefillin during morning prayer services and afterwards change to Rabbeinu Tam tefillin for a few minutes, in order to satisfy both opinions. Sephardi Jews follow Rashi's opinion. One can easily identify the difference between Rashi tefillin and Rabbeinu Tam tefillin externally, without opening the boxes, since the calf-hair threads bound around one of the passages in the tefillin shel rosh protrude outside the box in different places on either type of tefillin.
Another halakhic disagreement between Rabbeinu Tam and Rashi concerns the placement of the mezuzah. Rashi rules that it should be mounted on the doorpost in a vertical position; Rabbeinu Tam holds that it should be mounted horizontally. To satisfy both opinions, Ashkenazi Jews place the mezuzah on the door in a slanted position. Sephardi Jews mount the mezuzah vertically, per the opinions of Rashi, Maimonides, and the Shulchan Aruch.
Rabbeinu Tam is credited with writing Sefer Hayasher (םפר הישר). His brothers were the Rashbam (רשב"ם) and the Rivom (ריב"ם).
Rabbeinu Tam and his brothers, the Rashbam and the Rivom, as well as other Baalei Tosafos, were buried in Ramerupt. The unmarked, ancient cemetery in which they are buried lies adjacent to a street called Street of the Great Cemetery. In 2005, Rabbi Yisroel Meir Gabbai, who renovates and repairs neglected gravesites of Jewish leaders around the world, helped to determine the exact boundaries of the cemetery. In addition, a member of the Jewish religious community in Paris bought a house at the site and converted it into a beth midrash .