If you’ve ever examined the ‘batim’ of a ‘shel yad’ and a ‘shel rosh’ closely, you have probably noticed a big difference between them. The ‘bayis’ (sing. for ‘batim’) of the ‘shel yad’ is smooth, while the ‘bayis’ of the ‘shel rosh’ has three lines “etched” into three of the five “faces”. (The other two faces each have a large ‘shin’ letter on it – more on that later.) In truth, those “lines” are not really lines at all; they are spaces. Yes, you heard me – spaces between the four compartments of the ‘bayis’, which were pressed together to form a perfect cube.
At its root, the construct of the ‘bayis shel rosh’ is based upon two main criteria which derive from the Torah itself. On one hand (head, actually), the Torah refers to the ‘shel rosh’ as ‘totafos’ (see Devarim 6:8). Rashi (loc. cit.), based on the Gemara, explains that the word ‘totafos’ is a contraction of two words: ‘tot’ and ‘fot’, which each means “two” in a different language – “Katfi” and “Afriki”! The reason it is called this (i.e. two-two) is because it houses four ‘parshiyos’ (paragraphs), each one in its own ‘bayis’ (i.e. small compartment). On the other hand, another verse in the Torah refers to the tefillin as a ‘zikaron’ (a remembrance – see Shmos 13:9), which is a singular word. Which is it – four or one? The resolution is that the ‘shel rosh’ must be made of four compartments forged into one! (Note: Since the ‘shel yad’ is not referred to in the Torah as ‘totafos’, it does not have to have four compartments, even though it contains the same four ‘parshiyos’ as the ‘shel rosh’. Thus, the ‘bayis shel yad’ is a single large compartment.)
The big question – both in theory and in actual construction – is how does one merge four compartments into one? The answer is that there are several different ways that this can be accomplished.
Rather than “inventing the wheel”, please allow me to excerpt an informative exposition I found on Wikipedia (http://en.wikipedia.org/wiki/Tefillin), which outlines four different types of tefillin ‘batim’. Regardless of the type, the foundation for its construction is the same, as mentioned above – combining the aspect of “four” with the aspect of “one”.
Tefillin vary in quality, the way they are made, and in their halachic desirability. On the market today there are four types: -
Peshutim (Simple Ones) - These are made using several pieces of parchment to form the inner walls of the head tefillin, glued within a slit square to divide it into the four required compartments. If the inserts are glued incorrectly then these batim are not kosher for use. The parshiyot inside commercially bought peshutim are generally of very poor quality, and can often be invalid.
Peshutim Mehudarim (Superior Simple Ones) - These make the box of the tefillin out of a single piece as required. They are typically made with 32 mm sides to the boxes, which are quite small. However, goat skin is used to form lighter weight batim, which when finished look almost identical to the more expensive cowhide type, but they are not as durable. Dakkot (Thin Ones) - These 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, including the inner as well as the outer walls as well as the base, which is halachically desirable. Its thinness means that the tefillin can become halachically invalid relatively easily if knocked, or through normal wear and tear.
Gassot (Thick Ones) - These are made entirely out of a single piece of thick leather (usually with inserts to ensure they close flat). This requires the repeated use of several tons of pressure in industrial presses as part of a complicated but delicate production plan. The resulting batim are so durable and thick that they can be renewed even if seriously damaged and they typically last a lifetime. Gassot are made with boxes varying in size from about 20 mm per side to over 40 mm, though sides of 31-36 mm are considered standard. To produce Gassot the choicest cow-hide is used from the cheeks and the neck where it is the thickest. Thus only one pair of tefillin is produced for each head of cattle. After undergoing a softening process the leather is cut to size and left to dry slowly for at least three months. The box shapes are then formed through the appliance of considerable pressure and gradually the cubic shape starts to form in the skin.
Please go to the above web-page for a few diagrams which may help you visualize what you just read. If anyone finds a site with better pictures or diagrams, please let me know.
You may understand a bit better now why there is such a range of prices for tefillin!