The mezuzah is a small parchment scroll fixed to the doorpost of a Jewish home, and it fulfils a commandment written into the very text it contains.
The commandment on the doorpost
The scroll holds the first two paragraphs of the Shema, handwritten by a trained scribe on parchment, and those passages include the instruction to write these words on the doorposts of the house, so the mezuzah literally does what the text inside it commands. It is fixed to the right-hand doorpost as one enters, set at a slant, and it is placed on the doorposts of the rooms of a dwelling, not only the front door. Many touch it and kiss the fingers on passing, a gesture of recalling the words within.
Made and checked
The validity of the mezuzah rests on the scroll, which must be written correctly by hand with every letter intact; a decorative case with no valid scroll, or a printed text, does not fulfil the commandment, and observant households have the scrolls checked periodically for faded or cracked letters. The case that protects the scroll may be plain or ornate and often bears the letter shin for one of the names of God. The object marks the home as Jewish and places the central declaration of the faith at every threshold, so that entering and leaving are framed by the words affirming the oneness of God.