Does a chatzitza nullify the tefillin?
According to most Rishonim, a chatzitza between the arm/head and the tefillin nullifies the mitzva. Even a minute chazitza that interferes with only a small area of the tefillin presents a problem. A second opinion holds that a chatzitza does not disqualify the mitzva. Nonetheless, it is important that the shel yad be placed under the garments and not over them. Chazal understood from the wording of "it shall serve as a sign for you" that this sign is only for you and not [to wear on the outside] for others. The halacha follows the first opinion.
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