Checking Tefillin

Tefillin must be checked twice every seven years to ensure that they are kosher.

What are tefillin dakkot?
“Dak” means thin in Hebrew. Tefillin dakkot are made by stretching a thin layer of parchment over a structural base similar to the peshutim. This outer parchment forms the entire box of the tefillin, which is halachically preferable. Because of its thin design the tefillin can become halachically invalid more readily than tefillin gassot. Although until a century ago tefillin dakkot were actually the only type available, today it is almost impossible to obtain well-constructed tefillin dakkot. 

What are tefillin gassot?
“Gas” means “coarse” or “thick.” Tefillin gassot are made entirely out of a single piece of thick leather, from the cheeks and the neck of the cow, where the hide is thickest. Working such thick leather into a finely finished product requires the repeated use of several tons of pressure in industrial presses as part of a complicated, but delicate production process. The resulting battim (boxes) are so durable and thick they typically last a lifetime.

What is the difference between the different levels of Tefillin Gassot shown?
The boxes and straps are the same. The difference is the level of hiddur of the parchments, i.e. the beauty of the writing. Some sofrim (scribes) were graced with more attractive writing than others, and their work commands a higher price.