What the object is made of

Everything in the object comes from an animal that could have been eaten, and every part of it has to be made with the intention that it is for this — hide processed for tefillin, parchment written for tefillin, straps blackened for tefillin. Intention is a material property here: identical leather prepared without it is unfit.

The head-box's four compartments are drawn up as channels from a single base and pressed square while wet, so that the block is one piece of hide rather than four glued together; the shins are pressed in from the outside with a form. The straps are black on the outside by received law, and long — long enough for seven turns and three, with a tail.

The craft note worth keeping: the scrolls are unrepairable. A Torah scroll can be corrected. Tefillin parchment must be written in order, so a letter discovered missing cannot simply be added — adding it would put a later letter before an earlier one, and the scroll is finished. The most hidden component of the object is the least forgiving one, and nobody will ever see it.

A working pair is checked by opening it, which destroys the sewing and requires a scribe to close it again — so most pairs are opened perhaps twice in a lifetime, on the assumption that what was made properly has stayed that way.