Exodus 13:9

A sign upon thine hand] a figurative expression meaning that they were never to lose sight of this duty. In later times the Jews understood this injunction literally, and to this day at times of prayer they attach to their left arm and forehead small cases containing pieces of parchment inscribed with certain passages of the Law. These cases are called in NT. 'phylacteries': see further on Deuteronomy 6:8.