"And you shall write them on the doorposts of your house, and upon your gates..."
These words of the Sh'ma (deuteronomy 11:13-21) are the source for the essential Jewish tradition of the mezzuzah. The mezzuzah is a small scroll ('klaf' in Hebrew )on which the words of the Sh'ma have been handwritten. The first paragraph declares, "Hear, O Israel! The Lord our God, the Lord is One...", and continues with an individual's acceptance of God's sovereignty. The second paragraph alludes to all the good things that will come when the commandments are followed, "your days and the days of your children will be lengthened". The scroll is placed in a small box or container that is then attached, at an angle, to the doorposts of a house.
A kosher mezzuzah should be handwritten on parchment, and cannot contain any mistakes. The mezzuzah and case containing the scroll should be placed in the top third of the doorpost on every door in the house, except a bathroom, on the righthand side as you enter a room, or on the side without the hinges (although a mezzuzah can also be affixed to a doorway without a door, that connects two rooms. It should be placed at an angle so the top of the mezzuzah tips into the room as you enter. The slanted position resulted from a compromise between Rashi and his grandson. Rashi argued that the mezzuzah should be placed vertically, and his grandson argued horizontally.
Before affixing the mezzuzah to the doorpost, one should recite the following blessing:
Baruch Ata Adonai, Eloheinu melech ha-olam asher kidshanu b'mitzvotav vetzevanu leek-bo-ah mezzuzah.
Blessed are You, Lord our God, Master of the universe, Who has sanctified us with the Commandments, and has commanded us to affix a mezzuzah.