The Torah instructs placing tefillin (leather phylacteries worn during prayer) "between your eyes." Taken literally, this would mean on the bridge of the nose or the forehead directly above the space between the eyes. But the Mekhilta ruled that the phrase actually means the top of the head — well above the literal space between the eyes.
The proof comes from (Deuteronomy 14:1): "Sons are you to the Lord your God. You shall not gash yourselves, and you shall not make a bald spot between your eyes for the dead." This verse prohibits making a bald patch "between your eyes" as a mourning practice. But the rabbis understood from tradition and observation that the area where mourners would shave was the top of the head, not literally the space between the eyes — no one shaves between their eyes in grief.
The Mekhilta applies the interpretive principle of gezerah shavah: the same phrase — "between your eyes" — appears in both the tefillin passage and the mourning passage. Just as "between your eyes" in the mourning context means the top of the head, so "between your eyes" in the tefillin context means the top of the head.
This ruling places the head tefillin at the hairline or above, centered on the skull — not on the forehead or between the eyebrows. The distinction matters enormously. A tefillin box placed literally between the eyes would look and function very differently from one placed on the crown. The Mekhilta's reading, derived from a mourning law about bald spots, determined the appearance of Jewish prayer for all subsequent generations.