10:12 A left-handed person puts tefillin on his right arm, which is the weaker arm. If a person does all of his work with one hand but he writes with the other, the arm with which he writes is considered his strong arm and he puts the tefillin on the other one. An ambidextrous person puts tefillin on his left arm, just like a right-handed person does. Someone who was born right-handed and conditioned to write with his left hand, but he does everything else with his right hand, puts tefillin on his left arm. 10:13 The straps of the tefillin, both of the head and of the arm, must be no less than the length of a barleycorn (one-third of an inch). The straps that hang down on either side of the head must reach the navel; some say the one on the right should reach at least until the place of the circumcision. It is advisable to follow the stricter opinion in this matter. The strap of the arm tefillin should be long enough to wrap seven times around the arm, thrice around the middle finger, and enough times around the hand to secure it. If the strap of either the head or the hand tefillin breaks, one must show it to an authority for evaluation.