There are a number of ways to construct the two leather boxes – the shel yad and the shel rosh. The best and most difficult way is to mold a single piece of leather into a box shape. Another technique is to cut and glue leather into a box shape.
Molded leather, of course, is more work, more expensive and sturdier. These tefillin are generally referred to as Tefillin Gassot. Cut and glued tefillin are called Tefillin Peshutim or Tefillin Peshutim Mehudarim. The main difference is that Tefillin Peshutim Mehudarim boxes are formed out of a single piece of hide as required, whereas Tefillin Peshutim rely on a halachic leniency that glueing the leather into a box shape renders it a single piece.
A third type – somewhere in between Tefillin Peshutim and Tefillin Gassot – is called Tefillin Dakkot, which include a thin layer of leather stretched over the box to unify it. With Tefillin Dakkot this outer piece of leather forms the box of the tefillin, including both the inner and outer walls and the base, which is halachically preferable. However, because the leather is thin the tefillin can lose their shape relatively easily if banged or as a result of normal wear and tear.
What’s inside the tefillin boxes?
The tefillin boxes (called battim) house strips of parchment with four passages from the Torah written by a specially trained scribe, known as a sofer.
The arm tefillin contains all four sections written on a single piece of parchment (or klaf). In the head tefillin there are four separate compartments, one for each of the four.
To be kosher, the letters on the parchment must be whole and distinct. If ink has flaked off and the letters are broken or run into one other, the tefillin are not kosher. Cheap tefillin are written on parchment which has been treated to make it easier to write on, but the treatment hastens the decay of the letters. So even if you’re lucky and the letters are just barely kosher (because they were written as quickly as possible without violated the fundamental halachic requirements), they’re bound to decay in ten years.