​Detailed laws on Tefillin Batim / Tefillin Boxes 

There may be no separation between the body and the Tefillin Batim / Tefillin Boxes or the Retzuah / Teffilin straps that encircles the head or the upper arm which hods the Hand Tefillin box in place. Preferably there should not be any separation between the body and the rest of the Tefillin strap as well. This includes, shirt sleeves, watches or a Kipa. If medically necessary it is permitted to wear a medicinal patch or bandage under the Rezuah Shel Yad/Hand Tefillin Strap. One should consult a Rabbi if his arm is in a cast.

There should be no writing on the underside of the Tefillin Batim / Tefillin Boxes and Tefillin Retzuos / Straps. Some Rabbis consider long bangs to be a separation between the Tefillin and the head. A person should also take in to consideration that it is hard to keep the Tefillin in the correct place if it is resting on long thick locks of hair.

The underside of the Retzuos must rest against the arm and the head. No part of the Tefillin Retzuah / Tefillin Strap may be flipped over that the underside is visible.