The Letters
The ש shin appears on the Tefillin in two forms: on the left side of the one worn on the head, the letter is embossed with four splays , while on the right side, the standard three-headed version of the ש shin is used. The two forms infer the two kinds of Divine wisdom, the revealed and the concealed. The four-headed shin appears nowhere else.
The Knots
There is a knot on the head Tefillin straps. The knot that goes at the base of the skull is shaped like a ד (dalet). The arm Tefillin straps have a knot formed like a י (yod).
The Source
דברים פרק כח , י וְרָאוּ כָל -עַמֵי הָאָרֶׁץ כִׁי שֵם ה 'נִׁקְרָא עָלֶׁיךָ וְיָרְ אוּ מִׁמֶׁךָ:
“Then all the nations of the world will see that the Name of G-d is upon you, and they will fear you.”
The Gemara (Berachos 6a) says that this verse refers to the head Tefillin. Abaye states this is a requirement of the Torah to have the letter Shin on the head Tefillin (Halacha to Moshe from Sinai). Rabbeinu Bachaye explains that the “Name of G-d” refers to the letter Shin that is found on either side of the head Tefillin. Rashi (in many places) explains that not only the Shin of the head Tefillin is a Halacha to Moshe from Sinai, but also the Dalet and Yud in the straps.
Tosfos disagrees and maintains that only the Shin is a Halacha to Moshe from Sinai. However, the Dalet and the Yud in the straps are not included. Tosfos brings a proof that the Gemara holds that the straps are not even included in the Torah requirements from Tefillin (required to have but the format is from the Rabbis). Therefore, the letters also could not be included in the Halacha from Sinai.
One other opinion expresses that the Shin and Dalet (upon the head Tefillin) are from a Halacha to Moshe from Sinai (as evidenced from the Medrash that Moshe saw the head Tefillin of HaShem). The Yud in hand Tefillin was from the Rabbis in order to complete the holy name. Evidence to this opinion is that the Yud was removed to a distance in the hand Tefillin, to emphasize that is was not part of the Halacha to Moshe from Sinai.
The different names of Tefillin: pe’er, tiferes, and oz
Rabbeinu Avraham min HaHar writes that Tefillin are referred to as pe’er because it is said vrau kol amei haaretz ki sheim HaShem nikra olecho veyaru mimeka, “then all the peoples of the earth will see that the Name of HaShem is proclaimed over you, and they will fear you”, and the Gemora in Brachos states these are Tefillin shel Rosh. Possibly, the verse that states and they will fear you refers to Tefillin shel Rosh because they are visible, whereas the Tefillin shel yad are not visible. Why then is a mourner exempt from Tefillin shel yad which does not seem to be classified as pe’er?
The Meshech Chochmah at the end of Parashas Bo writes that we say in the prayer of Vehu Rachum on Monday and Thursday ad masai uzcho bashvi vsiferatecho beyad tzar, “until when will your strength be in bondage and Your Splendor in the hands of the enemy?” Oz refers to Tefillin shel yad, and tiferes refers to Tefillin shel Rosh.
The Meshech Chochmah seems to imply that tiferes-pe’er only refers to the Tefillin shel Rosh. Yet, the Meshech Chochmah himself in Parashas Beshalach writes that both oz and pe’er refer to Tefillin. Perhaps when oz and tiferes are written together, one can distinguish between the Tefillin shel yad and the Tefillin shel Rosh. Rashi in Ta’anis 16a and other Rishonim in Moed Katan, Kesuvos and Bava Basra seem to imply that pe’er refers only to Tefillin shel Rosh. It is also possible that there is a distinction between tiferes and pe’er.
The Mishna Berurah in Hilchos Tisha B’Av quotes the Medrash that states that the verse that states hishlich mishamayim eretz tiferes Yisroel, “He cast down from heaven to earth the glory of Israel” refers to Tefillin, and that is why we do not wear Tefillin on Tisha B’Av morning.
