Rabbeinu Tam grew up to become the foremost [[Halakha|halakhic]] authority of his generation and one of the greatest leaders of his time. He was one of the most famous of the [[TosafistsTosafist]]s. He acquired his name, ''Tam'', because his [[Bible|biblical]] namesake, [[Jacob]], is described as "''tam''", implying straightforward and truthful.
While the Tosafists' rulings tended to side with [[Ashkenazi Jews|Ashkenazi Jewish]] 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.