The location of the shel rosh
The Torah says that the shel rosh should be "a totafos between your eyes." By means of a gezeirah shavah-comparison to the prohibition of mutilating one's body over a meis, Chazal understood that 'between your eyes' refers to the place where hair grows. The expression 'between your eyes' also teaches that the tefillin should not only be placed above the hairline, but also aligned between the eyes.
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