Sinew string is known as gid. The sinews from the calf area of the cow or bull is removed and dried. The outer hard shell is removed, revealing many threads. These threads are pulled off and twined together by hand with an adhesive to form an extremely strong string. They are twined to form a long stretch of gid and wrapped up as a ball or around a piece of plastic for easy unraveling.

Some opinions require the gid to be made lishma, for the sake of the holiness of tefillin, which can be done by a Torah observant adult.

Cow's Tail
Before the tefillin are sewn, each of the four folded head tefillin parchments and the single, rolled arm tefillin parchment are wrapped with sa'aros, hairs from the tail of a cow. They are then tied with a single piece of plain parchment and again sa'aros are tied around them.