The placing of the Torah passages within the batim
Each passage of the hand Tefillin and of the head Tefillin is wrapped in parchment and is bound with a lock of hair from the tail of a calf.
The passages are inserted in the batim.
Extra care is taken with the head Tefillin to ensure that several of the hairs used to bind the passage of "If, then, you obey" will protrude beyond the titora.
Finally, the batim are stiched up with cow gut, making sure that hairs from a calf tail protrude into the stitching of the head Tefillin.