According to the Mishna (daf 28a) all four of the parshiyot, or chapters, of the Torah that are placed in tefillin (Shema and ve-haya im shamo’a, as in mezuzot, as well as the two other parshiyot where the mitzva of tefillin appears in Sefer Shemot, Chapter 13 – kadesh and ve-hayah ki yevi’akhah), must be included in order for the tefillin be valid. This ruling leads the Gemara to segue from its discussion of mezuzot to a discussion of tefillin.
The Gemara quotes baraitot that describe the differences between the tefillin shel rosh – the one placed on the head – and the tefillin shel yad – the one placed on the arm. The tefillin shel rosh is made up of four separate pieces of parchment on which the different parshiyot are written. These parchments are placed in four separate compartments of leather that are made together as one. Rashi describes how the soft raw hide is placed over a mold with four protruding “fingers” so that during the processing the leather is stretched out in the appropriate form. These four compartments are pressed together, and when finished there is a clear separation between them that can be seen even from the outside.
In contrast, the baraita teaches that in the tefillin shel yad all four of the parshiyot are written on a single piece of parchment and placed in a leather box that has a single compartment.
In the continuation of the Gemara, Rav Ḥananya quotes Rabbi Yoḥanan as teaching that although, if necessary, tefillin shel yad can be made into a shel rosh, a shel rosh cannot be made into a shel yad, since the tefillin of the head have a higher level of holiness, and the general principal is that we do not lower the holiness of a sanctified object.
Rashi suggests that the higher level of holiness ascribed to the tefillin shel rosh stems from the fact that two of the three letters of the holy Name shin-dalet-yod are on the shel rosh (the shin appears on the compartment and the dalet is on the knot at the back of the tefillin, while only the yod is made as a knot on the shel yad. Others disagree with this reasoning and argue that by definition the holiness of the head – the center of human intellect – is of greater import than that of the hand.