Deuteronomy 6:9. Thou shalt write them upon the posts of thy house, and on thy gates.
It was a common custom among the ancient Egyptians to write inscriptions on the doors of their houses. Besides the names of the dwellers, lucky sentences were written. The Mohammedans write passages from the Koran on their doors. “O God!” is written on some; “the Excellent Creator is the Everlasting,” is also seen. The modern Jews have in some places an arrangement equivalent to this. The passages in Deuteronomy 6:4-9 and 11:13-21, are written on one side of a piece of parchment which is prepared especially for the purpose, while on the other side is written שַׁדַּי, the Almighty. The parchment is then rolled up, so that the sacred name shall be on the outside, and is put into a reed or metallic cylinder, which has in it a hole just large enough to show the שַׁדַּי upon the parchment. This hole is covered by a piece of glass. Such a cylinder, with its parchment roll, is known by the name of Mezeuza, and is fastened to the right-hand door-post of every door in the house, so that it is in full sight, and may be touched or kissed as the dwellers in the house go in and out. The Jews from a very early period believed that the Mezeuza guarded the house against the entrance of diseases and evil spirits.