R. Eliezer says: "on your hand" — the height of the hand. You say "the height of the hand," but perhaps "your hand," literally, is intended. It is, therefore, written (Shemoth 13:9) "And it shall, therefore, be for you as a sign" — "for you as a sign" (i.e., on the upper arm) and not for others as a sign (i.e., not in the palm of your hand.)
R. Yitzchak says: "on your hand" — the height of the hand. You say "the height of the hand" but perhaps "your hand," literally, is intended. It is, therefore, written (Devarim 6:6) "And these things shall be upon your heart" — alongside your heart, i.e., upon the upper arm.