There is an ancient difference of opinion regarding the order in which the passages are inserted into the Batim: According to Rashi, the passage of Shema (“Here O Israel”) precedes that of “And it shall come to pass, if you hearken”, both in tefillin worn on the head and on the arm. According to Rabbeinu Tam, the order is reversed.
Our custom is to follow Rashi’s order. However, some people wear both Rashi and Rabbeinu Tam tefillin.

How to distinguish between Tefillin of Rashi and Rebbeinu Tam
The calves’ hair threads bound around the passage “And it shall come to pass” protrude outside of the Bayit Shel Rosh. One can easily distinguish between the two types of tefillin without opening the Batim according to the location of the protuding threads. (see pictures below)

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