The Torah requires wearing tefillin on the “weaker” of the two arms. The halachic authorities debate the question of how this status is determined. One view maintains that the arm used for writing constitutes the “strong” arm with respect to this halachah, whereas the other contends that the “strong” arm is the arm used for most activities.[126] We present in this chapter the halachic conclusions reached by the later authorities in this regard. A left-handed person should consult with his Rabbi for practical guidance, as numerous different factors may be relevant in determining his status with respect to tefillin.

1) A person who writes and performs most activities with his right hand wears his tefillin shel yad on his left arm, whereas a person who writes and performs most activities with his left hand places his tefillin shel yad on his right arm. One who wears the tefillin on the wrong arm does not fulfill the mitzvah.[127]

2) If a person writes with his right hand but performs all other activities with his left hand, or, conversely, he writes with his left hand but performs all other activities with his right hand, his status is determined based on the hand he uses for writing. If he writes with his right hand, he lays tefillin on his left arm, and vice-versa. However, since some authorities maintain that one’s status is determined in this case based on the hand used for other activities, the person should place tefillin after shaharit on the arm used for writing, without reciting a berachah, and recite Shema.[128]

3) Somebody who is ambidextrous, and performs all activities with both hands with equal efficiency, wears the tefillin shel yad on his left arm, as though he were right-handed. If, however, he can perform activities more efficiently with his left arm, he is considered left-handed even if he can also perform activities with his right arm.[129]

4) If one was born right-handed and trained himself to become left-handed, or if he suffered an injury to his right arm and as a result became left-handed, he is considered left-handed with respect to tefillin.[130]

5) If a right-handed person trained himself to write with his left hand, but still performs all other activities with his right hand, he lays tefillin on his left arm like other righthanded men.[131]

6) If a left-handed person trained himself to write with his right hand, but still performs all other activities with his left hand, he lays tefillin on his right arm.[132]

7) If a person writes with equal efficiency with both hands, but performs other activities with his left hand, he lays tefillin on his left hand. However, in order to satisfy all opinions after shaharit he should lay tefillin on his right arm without reciting a berachah, and then recite Shema.[133]