The Torah requires wearing the Tefillin Shel Rosh (the head Tefillin) "Bein Einecha" "in between your eyes." It is clear from the Gemara that this verse is not to be taken literally to mean that one places the Tefillin on his forehead in between his eyes. In fact, the Gemara remarks that people who accept the literal meaning of this verse are deemed heretics, as they misinterpret the Torah. The true meaning of this verse is that one places the Tefillin Shel Rosh on his head parallel to the space in between his eyes.
Where precisely must the Tefillin Shel Rosh be situated?
Halacha requires that the entire box of the Tefillin Shel Rosh be positioned above the individual's original hairline; no part of the box may protrude beneath the hairline. A man with a receding hairline may place the Tefillin Shel Rosh beneath his current hairline, provided that it does not extend beneath his original hairline. It should be emphasized that the Tefillin Shel Rosh must remain above the hairline, and not merely above the end of one's hair. Meaning, the Tefillin box may not extend past the root of one's hair, even if the hair grew and falls further down on his forehead.
How far back may the Tefillin be placed?
The Gemara mentions that the Tefillin may extend no further than the area on the head that is soft at childbirth. According to the Mishna Berura, this is determined by envisioning a line extending from the tip of one ear over the head to the top of the other ear. The Tefillin Shel Rosh may be positioned anywhere from the hairline until this imaginary line on the top of one's head.
It is therefore advisable for a person to position his Tefillin higher, rather than lower, on his head. Since he may place his Tefillin as far back as the point mentioned earlier, whereas in front it may not extend past the hairline, one should place his Tefillin considerably above his hairline to ensure proper compliance with these laws.
One should make a point to occasionally adjust the straps of his Tefillin Shel Rosh to ensure that its size allows for its proper positioning on the head. Tefillin straps tend to stretch over the course of time, and they therefore require periodic adjustments. Furthermore, after a person's hair grows or after he takes a haircut he will likely need to adjust the straps of his Tefillin Shel Rosh. Of course, someone who borrows Tefillin must ensure to adjust the straps to fit his head.
As mentioned earlier, the Tefillin Shel Rosh should be positioned in the center of the head, parallel to the space in between a person's eyes, and not to the right or the left of that point. It is therefore recommended to keep a small mirror with one's Tefillin so that he can ensure the proper position of his Tefillin Shel Rosh.
Summary: The entire box of the Tefillin Shel Rosh must be situated in between the person's original hairline and the point on top of the head which is soft at childbirth; one may occasionally have to adjust the straps of his Tefillin Shel Rosh so that it can be positioned correctly. The box should be positioned in the center of one's head, parallel to the space in between his eyes, and one should therefore have a small mirror so that he can ensure the proper position of his Tefillin Shel Rosh.
