The entire Tefillin must be placed above one's [original] hairline, and below the frontal ridge which is soft in babies.
Tefillin contain the 4 chapters in the Torah where Tefillin are written.
Applies to males, everywhere, every weekday.
Pasuk: "...it shall be as a sign between your eyes" (Devorim 6:8)
Source: The Chafetz-Chaim's Sefer haMitzvot haKatzar; Positive Mitzvah 9
Sunday, 21 Marcheshvan 5770