There are numerous types of tefillin covers, made of various materials. They protect the tefillin from damage (scratches, blows, etc.).

Therefore, covers should be chosen:
A) to fit the size of the Batim.
B) of a stiff durable material.
C) to cover the tefillin completely, top and bottom.

The tefillin for the arm also come with an additional small inner covering which should remain on the tefillin when they are being worn.

The covers are stamped, “Made of one piece of leather, including the hollow rim through which the strap is passed, (ma’abarta in Aramaic)”. One should not depend on this guarantee without personally verifying it through examination of the Batim.

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