A mezuzah is a small parchment scroll on which verses from Deuteronomy (6:4-9 and 11:13-21) are handwritten by a specially trained scribe called a sofer. Traditionally, Jews affix a mezuzah within a container on the doorposts of their homes. Mezuzot can also be found attached to the entrance gates of the Old City of Jerusalem.
To fulfill the commandment of making a mezuzah in accordance with Jewish law, the parchment must come from the skin of a kosher animal (usually a cow) following shechita, or ritual slaughter. Preparing the animal skin takes several weeks; most of this consists of treatment with lime and other depilatory chemicals in addition to stretching over a frame. To inscribe letters on the parchment, the scribe uses a feather quill from a kosher bird (most often a goose or a turkey) as well as shiny black ink made from a mixture of gall nuts, gum arabic resin and copper vitriol.
Several additional rules apply to transcribing the appropriate Biblical passages to the parchment of the mezuzah. Most notably, adjacent letters may not touch. If two letters touch or smear together, this ritually nullifies the mezuzah. Also, certain Hebrew letters are written with crowns above them. Once the mezuzah is placed in a decorative cover and affixed to the doorpost, it should be checked by a scribe twice every seven years to ensure it remains valid for ritual use.