The Order of the Parshiyos in Tefillin

There is an ancient difference of opinion about the order in which the passages (parshiyos) are placed in the batim of the tefillin. According to Rashi, the passage of Shema ("Hear, O Yisroel") comes before "And it shall come to pass, if you hearken," both in the tefillin of the head and in the tefillin of the arm. According to Rabbeinu Tam, the order is reversed. Some people wear both the tefillin of Rashi and those of Rabbeinu Tam.