Reference for Chullin 17:4
תלמיד חכם צריך שילמוד ג' דברים
R'Hanania B'Shelemia said in the name of Rab, He must also learn the art of forming the knot of the Tefillin,<span class="x" onmousemove="('comment',' V. Glos. When properly tied the knot in the Tefillin worn on the head forms the shape of the Hebrew letter Daleth and that of the letter Yod in the Tefillin worn on the hand.');"><sup>2</sup></span>
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