Position of Tefillin Shel Rosh

Question

Why is it that we wear our tefillin specifically above our hairline? Is there any special reason for that part of the head? I recently heard the answer from a Rabbi however I couldn't recall. I am looking forward to the answers with sources.

Answer 1

Talmud Bavli, Menachot 37a:

תנא דבי מנשה על ידך זו קיבורת בין עיניך זו קדקד היכא אמרי דבי רבי ינאי מקום שמוחו של תינוק רופס

The school of Menashe taught with regard to the verse: “And you shall bind them for a sign on your arm, and they shall be as frontlets between your eyes” (Deuteronomy 6:8): “On your arm”; this is the bicep. “Between your eyes”; this is the crown of the head. The Gemara asks: Where exactly on the crown of the head are the phylacteries placed? The school of Rabbi Yannai say: Phylacteries are placed on the place where the bone above the baby’s brain is soft after birth.

(Translation/elucidation courtesy of sefaria.org)

This is explained in more detail on the next page of Menachot:

גובה שבראש מנלן דת"ר בין עיניך זו גובה שבראש אתה אומר זו גובה שבראש או אינו אלא בין עיניך ממש נאמר כאן בין עיניך ונאמר להלן (דברים יד, א) לא תשימו קרחה בין עיניכם למת מה להלן בגובה שבראש מקום שעושה קרחה אף כאן בגובה של ראש מקום שעושה קרחה

With regard to the statement of the baraita that the phylacteries of the head are donned on the upper part of the head, the Gemara asks: From where do we derive this? As the Sages taught: “Between your eyes” (Exodus 13:9); this is the upper part of the head. Do you say that this is the upper part of the head, or is it only literally between your eyes? It is stated here: “Between your eyes,” and it is stated there: “You shall not cut yourselves, nor make any baldness between your eyes for the dead” (Deuteronomy 14:1), Just as there, the phrase “between your eyes” is referring to a place on the upper part of the head, as that is a place where one can render himself bald by removing his hair, so too, the place where phylacteries are donned is on the upper part of the head, a place where one can render himself bald.

(Translation/elucidation courtesy of sefaria.org)

Furthermore, we learn in Megillah 24b:

נתנה על מצחו או על פס ידו הרי זו דרך המינות

If one placed the phylacteries worn on the head on his forehead, and not in its proper place above his hairline, or if he placed the phylacteries worn on the arm on his palm, and not on his bicep, this is the way of the heretics, i.e., those who reject the tradition of the Sages with regard to the proper placement of the phylacteries.

(Translation/elucidation courtesy of sefaria.org)