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Mesartait in one of the 39 melachot, even though it is not listed in the Mishna on 73a.
What is Mesartait?
Rashi explains that mesartet means etching lines into hides so that they could be cut to the correct measurement. Mesartet isn’t specific to hides and applies to scoring any material in order for it to be cut precisely or written on afterwards. Similarly, the Rambam (Shabbos 11:17) writes that drawing a straight line on a log so that it can be cut evenly is a Toldah of mesartet. Additionally, the Rambam Pirush Mishnayot 7:2 writes that mesartet is done in preparation for writing upon hides.
- Rabbi Mayer Twersky (Shabbos shiur 26) suggested another approach. Interestingly, the Rambam (Shabbos 7:1) places mesartet right after kotev, writing, and not in the context of preparing hides. It follows that the Rambam (Shabbos 11:17) writes that the minimal amount one would need to violate in order to be obligated a chatat is etching a line long enough to write above it 2 letters, as 2 letters is the minimal size for the melacha of kotev. Additionally, the Rambam Pirush Mishnayot 7:2 writes that mesartet is done in preparation for writing upon hides. It seems that the Rambam understood mesartet to be etching lines into parchment so that you can write neatly, whereas Rashi understood mesartet to be etching lines into hides for them to be cut correctly.
In the Mishkan
- According to Rashi, it is easy to understand where mesartet was used in the construction of the mishkan; when they had to cut the hides to match one another, they would first score them. However, according to the Rambam, there is a question where in the mishkan was the Av melacha of mesartet used. Were the kohanim really so careful about writing down where each beam would be placed, that they would first etch lines before writing? Some suggest that mesartet, according to the Rambam, was used in the construction of the bigdei kehuna as they would need to etch lines into the tzitz and ephod so that the writing upon them would come out straight.
- The Gemara Shabbos 75b states that salting and tanning the hides are the same melacha and can’t be considered independent melachot. Because of this, the gemara replaces one of them with mesartet in order to account for 39 melachot exactly.
- Torat HaMelachot (vol 7, p. 150) quoting from He'ir Yosef