1) The Torah requires wearing the tefillin shel yad on “ידכה,” which literally means “your arm,” but can also be read as “יד כהה,” “the weak arm.” As such, a person defined by halachah as right-handed should wear the tefillin on his left arm, whereas somebody defined as left-handed wears the tefillin on his right arm. (The halachic definitions of “righthanded” and “left-handed” are explained in the section entitled “Defining the Weak Arm.”) One who wears the tefillin shel yad on the wrong arm does not fulfill the obligation.[105]

2) One should place the tefillin shel yad on upper part of the arm, on the lower half of the muscle, meaning, the half closer to the elbow.[105*]

One who placed the tefillin on the half closer to the armpit has nevertheless fulfilled his obligation, so long as the tefillin is situated on the elevated part of the upper arm.

However, if one wears his tefillin even slightly lower than (shown below) or above the muscle, he does not fulfill the obligation of tefillin and is considered as though he did not wear the tefillin shel yad at all.[106]

Therefore, one should not wear very large tefillin, as part of the tefillin will likely extend either above or below the area where the tefillin is to be worn. If one has access to only very large tefillin, he should let it extend to the upper half of the muscle, near the armpit;

this is preferable to allowing part of the tefillin to extend beneath the muscle, which would invalidate the mitzvah altogether and render the berachah a berachah le’vatalah (berachah recited in vain).[107]

3) The tefillin should be tilted slightly towards the body, such that when one drops his forearm the tefillin is situated near his heart, in fulfillment of the verse, “These words that I command you this day shall be upon your heart” (Devarim 5:6).[108]