For the source text click/tap here: Menachot 35
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Although the requirement that tefillin be made square is clearly presented by the Gemara as a halakha le-Moshe mi-Sinai, nevertheless, it appears from the discussion of the rishonim that this was only viewed as a necessity with regard to the tefillin shel rosh, but that the single parchment of the tefillin shel yad – tefillin worn on the arm – could be placed in a cylindrical leather covering if placed on a square base. Evidence of this practice can be found by examining the tefillin found in the Cairo geniza and in other illustrated manuals from the Medieval period.
We explore further the Halacha and especially the most arresting anthropomorphic images in rabbinic literature is the claim that God wears tefillin—and more radically, that God showed Moses the knot of His tefillin. In Menachot35b, the Gemara reads the enigmatic verse “Va-hasiroti et kappi v’ra’ita et achorai” (Exod. 33:23) not as a metaphysical abstraction but as a concrete pedagogical act: God reveals to Moses the acherai, His “back,” identified specifically as the knot of the head-tefillin.