The parchment scroll, written in the Torah's original Hebrew, contain two parshiot (Torah sections): Deuteronomy 6:4-9 and Deuteronomy 11:13-21.
These verses are the only inscription on the parchment scroll (except for the Sha-dai inscribed on the reverse side) and emphasize our belief in the Oneness of G-d.
The scroll, of handmade parchment from a kosher animal, is inscribed in black ink with a quill pen by a specially trained, religiously devout scribe known in Hebrew as a sofer. The sofer concentrates intensely and writes with special Hebrew characters in a beautiful calligraphic hand.
A mezuzah has 713 letters. Every letter has numerous laws pertaining to its form. In order for mezuzahs or tefillin to be written in accordance with all of the laws, it must thus meet thousands of requirements.
If even one of the 713 letters in a mezuzah is missing or shaped incorrectly, the mezuzah is rendered invalid, the Mitzvah is unfulfilled, and the blessing recited over it is in vain. Even the best scribe is human and subject to error. While some errors may be corrected in accordance with Jewish law, others cannot.
Mezuzahs and tefillin, even when written by the most expert of scribes, have many possibilities of being unfit. They must therefore be inspected before being purchased.
Furthermore, mezuzahs and tefillin, like anything else subject to the ravages of time and elements, can deteriorate. It therefore behooves us to inspect our mezuzahs and tefillin from time to time, either for the purpose of determining their fitness or for making preventative corrections. The halacha sets the requirements for periodic inspection of the mezuzah at every 3½ years.