Where precisely on the back of the head should one position the Dalet-shaped knot of the Tefillin Shel Rosh (Tefillin worn on the head)?
The Shulchan Aruch rules that at least part of the knot should be touching the "Etzem Ha'gulgolet," the bone directly beneath the skull on the back of the neck. Underneath the skull on the back of the neck there is an indentation, which one can feel when touching the back of his neck. Right above that indentation is a round bone, and the knot of the Tefillin Shel Rosh should be at least partially touching that bone. Many people mistakenly wear the Tefillin Shel Rosh with the knot situated considerably lower on the neck; a person must ensure that the knot is high enough that at least part of it rests on the bone directly underneath the skull.
On a different issue related to Tefillin, the Shulchan Aruch writes that while wearing Tefillin, one should periodically touch them in order to remind himself that he wears Tefillin and must therefore maintain an aura of sanctity due to their uniquely sacred quality. Indeed, common practice is to touch one's Tefillin and then kiss his hand from time to time during the prayer service, so as to remain cognizant of the sanctity of Tefillin.
Halacha requires that one should first touch the Tefillin Shel Yad (Tefillin worn on the arm) and only then touch the Tefillin Shel Rosh – the same sequence that is followed when putting Tefillin on in the first place. However, if a person's hand happens to be near his head when he decides to touch his Tefillin, he should first touch his Tefillin Shel Rosh. If he would first touch the Tefillin Shel Yad, he would appear as passing over the Mitzva of Tefillin Shel Rosh, which would constitute an insult, of sorts, to the Mitzva.
Summary: The Dalet-shaped knot of the Tefillin Shel Rosh should be worn such that it at least partially touches the bone directly underneath the skull on the back of the head, right above the indentation on the upper part of the back of the neck. One should touch his Tefillin from time to time while he wears them; one first touches the Tefillin Shel Yad, unless his hand happens to be situated near the head, in which case he first touches the Tefillin Shel Rosh.
