A knot with which one ties a strap to the camel’s nose ring and a knot with which one ties a rope to the ring fixed to the bow of a ship, with regard to liability to bring a sin-offering, there is none; however, there is a rabbinic prohibition. And there are knots that are permitted to be tied on Shabbat ab initio. And which are these? The knot that a woman uses to tie the opening of her robe.
מפתח חלוקה. פשיטא! לא צריכא, דאית ליה תרי דשי. מהו דתימא: חדא מינייהו בטולי מבטיל [לה], קא משמע לן.
We learned in the Mishnah: A woman may tie the opening of her robe on Shabbat. The Talmud states: This is obvious, as the knot is meant to be untied and is therefore not permanent. The Talmud explains: It is only necessary to state this halakha in a case where the robe has two laces with which to tie the opening. Lest you say that one of them may become void because the woman can remove the garment even with one string, leaving the one not untied a permanent knot, it therefore teaches us that neither knot is considered permanent.
וחוטי סבכה. פשיטא! לא צריכא, דרויחא לה. מהו דתימא: מישלף שלפא לה, קא משמע לן דאשה חסה על שערה ומישרא שריא לה.
We learned in the Mishnah: And a woman may tie the strings of her hairnet on Shabbat. The Talmud states: This is obvious. The Talmud clarifies the matter: It was only necessary to state this halakha in a case where the hairnet is tied loosely on her head. Lest you say that she sometimes removes it without untying it and the knots remain, the Mishnah teaches us that a woman is protective of her hair and avoids pulling it out, and therefore she unties the hairnet to avoid damaging her hair.