There are two main types of Batim:
A. From one piece of leather (“Or Echad”)
B. Simple Batim (“Peshutim”)
A. One piece of leather
These Batim are made of very thin sheepskin which is stretched on a special form and strengthened on the inside with additional pieces of sheepskin. The dye used is mixed with a thin coat of plaster covering the Batim.
How to choose sheepskin Batim
It is essential that the tefillin form a perfect square, top and bottom, and that the grooves of the Bayit Shel Rosh reach the seam (see the Batim).
B. Simple Batim
These Batim are made of several thick hides stuck together. Halachically, their status is questionable, and some authorities disqualify them altogether.
How to Choose Sheepskin Batim
It is essential that the tefillin form a perfect square, top and bottom, and that the grooves of the Bayit Shel Rosh reach the seam. (see the Batim).
Cowhide (gassot) < > The tefillin straps (retzu’ot)