Shabbos, Chapter 15, Mishnah 2. In the previous Mishnah we learned that there are permanent knots for which one is liable - such as a camel drivers' knot and a sailors' knot. In contrast to those, our Mishnah opens: "Yesh lecha kesharim she'ein chayav aleihen k'kesher hagamal uk'kesher hasapanin" - there are types of knots that are not comparable to those knots, and one is not liable for them.
The Gemara's explanation - what is the Mishnah referring to:
The Mishnah itself does not give examples, and it is the Gemara that explains: the reference is to knots made for a certain period of time and not permanently. Since they are not permanent, one is not liable for them by Torah law, but since they are meant to last for some time, they are forbidden rabbinically.
An example of this: the knot with which one ties the rope to the ring in the camel's nose, or tying the rope to the ring on a ship in order to moor it in the harbor for a certain period. Such a knot is meant to last for a limited time and is not permanent, and therefore it does not carry Torah liability, though it is forbidden rabbinically.
Knots that are permitted even at the outset:
From here the Mishnah lists knots that may be tied on Shabbos even at the outset, since they are regularly tied and untied:
"Kosheres ishah mafteach chalukah" - tying the opening of one's garment.
"V'chutei sevachah" - the strings and straps that extend from a hair net.
"V'shel pesikya" - the knots tied in the belt of the undergarment.
"U'retzuos man'al v'sandal" - the straps of a shoe and a sandal.
"V'nodos yayin v'shemen" - wine and oil skins were made of leather, and their mouths were closed and tied shut. The novelty the Mishnah is teaching us: even though they have two knots, and one could remove the wine or the oil by opening only one of them, so we might have been concerned that the second would remain permanently tied - nevertheless it is permitted to open them, because the usual practice is to open both.
"U'kederah shel basar" - the same applies to a pot of meat, where it was customary to tie a piece of cloth over its mouth in order to preserve the heat. Even though one could remove the food without untying the knot, since people are accustomed to untie it, it is not a permanent knot, and it may be opened on Shabbos.
Rabbi Eliezer ben Yaakov says: "Kosherin lifnei beheimah bishvil shelo teitzei" - it is permitted to tie a rope across the opening of a pen or a barn so that the animal will not go out. Here too there is a novelty: one could untie just one side, and nevertheless, since people are accustomed to untie the knot, such tying is permitted on Shabbos.
Tying a bucket - what is permitted and what is forbidden:
The Mishnah says: "Kosherin d'li b'fesikya" - one who sees a bucket near a well may tie it with his belt, since a person does not leave his belt there permanently, because he needs it as a belt to hold up his clothes. For this reason the knot made with it is not a permanent knot. But "lo b'chevel" - with a rope it is forbidden, out of concern that he may leave it there.
Rabbi Yehudah is lenient and permits tying it even with a rope. The Gemara clarifies that even Rabbi Yehudah's leniency was stated specifically regarding a weaver's rope, which he needs for his craft, and therefore he too will not leave it there permanently. The Sages decreed against a weaver's rope on account of an ordinary rope, while Rabbi Yehudah does not make such a decree. But all agree that with an ordinary rope there is a concern that he may leave it there and it will become a permanent knot.
Rabbi Yehudah's principle:
Rabbi Yehudah stated a principle: "Kol kesher she'eino shel kayama - ein chayavin alav". Any knot that is meant to last only temporarily does not carry liability. This is the basis behind his ruling regarding a weaver's rope used at the well: since even an ordinary rope, as long as it is not made to be permanent, carries no Torah liability - there is no room to decree against a weaver's rope on account of an ordinary rope.
In summary: In this Mishnah we learned the other side of the laws of tying - knots for which one is not liable. First, knots meant to last for a limited time (such as tying the rope to the ring in the camel's nose or to the ring on a ship), which carry no Torah liability but are forbidden rabbinically; and then knots that are regularly tied and untied - the opening of a garment, the strings of a hair net, the belt, the straps of a shoe and a sandal, wine and oil skins, a pot of meat, and tying a rope in front of an animal - which are permitted even at the outset. We also examined tying a bucket with a belt but not with a rope, Rabbi Yehudah's dispute regarding a weaver's rope, and the great principle: any knot that is not permanent carries no liability.