Jewish law governs not only the sofer's script but the physical materials a kosher Torah scroll, mezuzah, or set of tefillin must be made from, and a working sofer needs practical mastery of these requirements alongside calligraphic skill. The parchment, called klaf, must be prepared from the skin of a kosher animal according to specified processes, and the ink traditionally combines gall nuts, gum arabic, and other components into a permanent black compound that will not fade or smear, since a Torah scroll is expected to remain legible for use over generations.
Sofrim write with a quill, traditionally a turkey or goose feather, cut and shaped by the scribe himself to produce the fine, consistent strokes the required script demands, and a single significant error, an incorrectly formed letter, a letter touching another that must stand separate, or certain kinds of omission, can render an entire scroll unfit (pasul) for ritual use until corrected or, in serious cases, permanently. Because of the stakes involved, completed scrolls, tefillin, and mezuzot are increasingly checked using specialized computer software that scans the text for irregular letter shapes and other defects, a modern supplement to, rather than a replacement for, a trained sofer's own review.