Here too, it can't go below  the end of the hairline, and in the back, there's a small  bone in the back of the skull where this knot has  to be placed on and not lower down  on the nape since the tefillin are supposed  to be situated on the head, and if it goes down to the  indentation below the bone, then it's no longer the head,  that's already called the nape, the nape that belongs  to the neck, so therefore it is no longer considered  wearing tefillin on the head. 