Where precisely on the arm should one position his Tefillin Shel Yad? Halacha requires that the entire box of the Tefillin Shel Yad be located on the muscle, the part of the arm between the elbow and shoulder that protrudes upwards when one bends his arm. If any part of the Tefillin Shel Yad extends off the muscle in either direction – be it downwards towards the elbow and upwards towards the shoulder – one does not fulfill the obligation of Tefillin. Those with particularly large Tefillin must see to it that no part of the box extends off the muscle in either direction. Where exactly on the muscle should the Tefillin Shel Yad be placed? The Gaon of Vilna (Rabbi Eliyahu of Vilna, Lithuania, 1720-1798) was of the opinion that no distinction exists between the various areas on the muscle; there is no preference as to where precisely on the muscle one should position his Tefillin. However, the Shulchan Aruch and Mishna Berura maintain that optimally the Tefillin should be placed on the lower half of the muscle, the part closer to the elbow. It should go without saying that if by positioning the Tefillin on the lower half of the muscle one runs the risk of the Tefillin extending off the muscle, even slightly, he should place the Tefillin on the middle of the muscle to prevent this from occurring. When possible, however, one should endeavor to position the Tefillin specifically on the lower half of the muscle. Preferably, one should turn the Tefillin slightly inward, towards the heart, such that when he relaxes his arms and lets them fall to the sides of his body, the Tefillin touches the area near the heart. It must be emphasized that the Tefillin should be turned inward only slightly; if the box touches the heart area even when one's arm is bent, then the Tefillin is turned too far inward, and is thus out of proper position. In conclusion, then, the Tefillin Shel Yad should be position on the arm muscle, preferably on the lower half, closer to the elbow, but no part of the Tefillin may extend even slightly below or above the elbow. The box should be tilted inward towards the body, such that it touches the heart when one's arms are straight. It should be noted that Tefillin constitutes one of the most important Mitzvot Asei ("positive" commandments) in the Torah. One who does not know how to wear the Tefillin properly can go an entire lifetime without ever fulfilling this special Mitzva, even if he wears it each and every weekday. It is thus imperative for one to ensure to comply with these laws and see to it that his Tefillin are positioned properly.