381. The Side to Write On

Tefillin, Mezuzah v’Sefer Torah 1:8

It is a halacha l’Moshe MiSinai that a sefer Torah is written on g’vil on the side where the hair had grown. Tefillin are written on klaf on the side of the flesh. A mezuzah is written on duchsustos on the side of the hair. If one writes on klaf on the side of the hair, or on g’vil or duchsustos on the side of the flesh, it is invalid.

Tefillin, Mezuzah v’Sefer Torah 1:9

Even though it is a halacha l’Moshe MiSinai that a sefer Torah is written on g’vil, a sefer Torah written on klaf is also acceptable. G’vil was only specified in order to exclude duchsustos. A sefer Torah written on duchsustos is invalid. Similarly, a mezuzah written on klaf or g’vil is also acceptable; duchsustos is merely preferable.