The shape of the knots and the manner in which they are tied
There is a knot in the straps of the head Tefillin in the part that is behind the head of the person who has put on the Tefillin. This knot is either in the shape of the letter dalet (the English L) or square, according to family tradition. The knot in the strap of the hand Tefillin is in the shape of the letter yud.
The strap of the hand Tefillin is wound around the arm and the hand. According to the Beit Yosef tradition, this strap is wound seven times around the arm, from the outside in, towards the direction of one`s heart. According to the Ari tradition and the practice of Eastern Jews, the strap is wound seven times from the heart outwards. Ashkenazi and Eastern Jews also differ in the winding of the strap around the hand. In the Ashkenazi tradition the straps are wound around the hand, in the shape of the letter shin. Thus the knots on the head Tefillin and the hand Tefillin, together with the straps on the palm, form the word Sha-dai, one of the names of the Almighty. Eastern Jews wind the straps around the arm and the hand in a different manner (see the illustrations).