The Torah commands placing tefillin (leather phylacteries worn during prayer) "upon your hand." But where exactly on the hand? The Hebrew word yad can mean the entire arm from shoulder to fingertips. The Mekhilta ruled that "your hand" means the upper arm — the bicep area — not the literal palm or hand.
The reasoning uses an analogy between the two tefillin placements. The Torah prescribes tefillin for both the hand and the head. For the head, the phrase "between your eyes" does not mean literally between the eyes — it means the top of the head, the highest point (as the Mekhilta demonstrates elsewhere). Just as "head" refers to the height of the head, "hand" refers to the height of the hand — which is the upper arm, the part of the limb closest to the body and highest when the arm hangs naturally.
This interpretation has governed Jewish practice for millennia. The tefillin shel yad is wound around the upper arm, with the box positioned on the bicep facing the heart. The palm, the forearm, the wrist — none of these qualify.
The Mekhilta's method here is characteristic of Tannaitic legal reasoning. Rather than relying on tradition alone, the rabbis constructed a logical argument from the internal structure of the Torah's own language. The same interpretive principle that locates the head tefillin determines the arm tefillin. The two commandments illuminate each other, and neither can be understood in isolation.