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There are several parameters that dictate the grade of a Tefillin to be less or more "Mehudar"
also see: How to put on Tefillin - Tefillin
- The scroll
The better the writing of the Tefillin scroll or the leather on which the scroll is written, the better the Tefillin.
- Quality control (Hagahah)
The level of quality control (Hagahah) the scroll goes through before it is considered Kosher.
- The boxes (Batim) leather
The box of the Tefillin could be defined as Tefillin Dakot (thin) which is made of sheep leather, or Tefillin Gasot (thick) which is made of Ox or cow leather. Tefillin Gasot is considered to be higher in quality.
Boxes come in small to very large
- The boxes (Batim) structure
The boxes that contain the scrolls may be constructed from one or more parts. If the box is made as one part, it is considered best, and is caleld in Hebrew "Or Echad" (one leather).
- The Straps
Retzuot - Or Tachton
Retzuot - Or Elyon
Retzuot - Avodat Yad
The Bible tells us: And you shall bind them as a sign on your arm, and they shall be as frontlets on your head between your eyes" (Deuteronomy 6:8.) Though of course the details are much more complex than this.... The Torah tells us in four places that we should bind the Tefillin.
The four place this is mentioned are:
and Exodus 13:11-16
Each of these places is a separate chapter in the Torah