Now we get to one of the “weak spots” of the mitzvah of wearing tefillin: the proper placement of the ‘bayis’ of the ‘shel rosh’. Unfortunately. many people are either unaware, or less than vigilant about the “lower boundary” prescribed for this mitzvah object. Every person is born with a natural hairline. Even if one’s hairline recedes later in life, and even if one becomes completely bald, the lower boundary is still the original natural hairline. This means that if the lower edge of the ‘bayis’ is hanging at all below that “line”, the tefillin is not in its proper place! (And we know what that means!)
In terms of the lower boundary of the ‘kesher’ (knot) in the back of the head, it should also not extend below the back hairline. It should sit right in the little crevice under the base of the skull.
For both issues mentioned above, it is advisable that a person have his ‘shel rosh’ checked out by a competent “tefillin agent”, i.e. someone who knows how to adjust the knot and make the loop smaller or larger if necessary. Even if one once had it fitted, it is quite possible that over time the leather has become stretched, causing the tefillin to hang too low in the front or back.
Until now we have discussed the vertical placement of the ‘shel rosh’. In terms of the horizontal placement, the Torah states, “and they shall be ‘totafos’ between your eyes.” We know from the Oral Torah that this verse does not mean “between your eyes” in a vertical sense (as we have just discussed above with the hairline being the lower boundary.) Rather, it means in a horizontal sense, i.e. the tefillin should be centered – both the ‘bayis’ and the ‘knot’.
In the course of wearing the tefillin, it is very common for them to shift – both vertically and horizontally. The halacha requires us to check them periodically to be sure that they remain in their proper place.
Most tefillin ‘retzuos’ (straps) are painted black on one side and left uncolored on the other. The rule is “black side up”, particularly the part of the ‘retzuos’ that attach the ‘bayis’ of both the ‘shel yad’ and the ‘shel rosh’ to the body.
Question: How long should the ‘retzuos’ of the ‘shel rosh’ hang down – i.e. after the knot? Answer: Minimally, two handbreadths. Customarily, however, the right one hangs down at least to below the waist and the left one to the naval.
Question: How wide should the ‘retzuos’ be?
Answer: A minimum of 9-11 millimeters, which is the numerical equivalent of the Talmudic standard – the width of the barley or wheat kernel.
Finally, the ‘bayis’ of the ‘shel rosh’ should be revealed (i.e. uncovered by the talis) and the ‘bayis’ of the ‘shel yad’ should be covered (i.e. by a sleeve). The source for these two particulars is the verse itself. Regarding the ‘shel rosh’, the Torah states that when the nations of the world see us in our ‘tefillin shel rosh’ they will fear us. Thus, they must be seen. Regarding the ‘shel yad’ the Torah states “and they shall be a sign for you upon your arm”, and the Talmud expounds, “For you, not for others”!