Reference for Menachot 69:3
ואמר רב חננאל אמר רב תיתורא דתפילין הלכה למשה מסיני אמר אביי מעברתא דתפילין הלכה למשה מסיני ואמר אביי שי"ן של תפילין הלכה למשה מסיני
We must rather say that there Is no such distinction.<span class="x" onmousemove="('comment',' And any change in the order of the portions will render the tefillin invalid.');"><sup>3</sup></span> R'Hananel also said in the name of Rab, The underside<span class="x" onmousemove="('comment',' Each tefillah, it must be remembered, is in the form of a square leather box upon a base, that of the hand consisting of one compartment and of the head of four compartments. In order to obtain the necessary shape (usually in the form of a cube) a mould or frame is used over which the skin whilst moist and pliable is tautly stretched. On being removed from the frame the skin is cut around to an equal length on three sides, whilst on the fourth side there is left a long strip of skin which, after allowing for a projection on this fourth side in order to provide a loop or a duct through which the straps are passed, is bent under the whole box so as to form the underside or the base of the tefillah. After inserting the necessary texts into the several compartments the base is stitched carefully to the extremities of the box on three sides.');"><sup>4</sup></span> of the tefillin is a law given to Moses at Sinai.