403. Positioning the Arm Tefillin

Tefillin, Mezuzah v’Sefer Torah 4:2

Tefillin of the arm are tied on one’s left arm at the muscle where the arm bulges between the shoulder and the elbow. Accordingly, if one were to press his arm to his ribs, he would find his tefillin opposite his heart, as per Deuteronomy 6:6, “these words... shall be upon your heart.”

Tefillin, Mezuzah v’Sefer Torah 4:3

If someone places his tefillin on his palm or on his forehead, he is acting like a Sadducee. (The Sadducees rejected the oral law.) If a person makes his tefillin rounded, he does not fulfill the mitzvah at all. A left-handed person puts tefillin on his right arm because to him it is like his left arm. An ambidextrous person places tefillin on his left arm. The places where tefillin are placed were communicated to us as part of the oral law.