On [[Shabbat]], there are certain types of knots that one may tie or untie and some that one may not tie or untie. See below for details and practical applications.
== The Basics of Koshair ==
= Melechos Koshair and Matir (Jonathan Tavin) =
'''Where was it in the Mishkan?''' The gemara<ref>Shabbat 74b</ref> explains that Koshair was performed in the construction of the Mishkan by the trappers of ''chilazon'' who would tie knots in their nets in order to adjust their nets.
'''Definition''': There is an important machlokes Rishonim as to the definition of a knot that would warrant a chiyuv min hatorah. According to Rashi<ref> Shabbos 112a s.v. b’dirabanan</ref> and Rosh<ref> Shabbos 15:1</ref>, as long as the knot is intended to stay tied forever or for a long time, one is chayav min hatorah for tying it. According to Rambam<ref
name="ftn151"> Shabbos 10:1</ref> and Rif<ref name="ftn152"> Shabbos 41b</ref>, not only must one intend to keep it tied for a long time to be chayav min hatorah, it must also be defined as a ''kesher uman'' —professional knot, such as a sailor’s or camel-driver’s knot.
'''Permanence''':There are different opinions as to what constitutes a ‘long time.’ The Kol Bo<ref> Cited in Rama 317:1</ref> says that a knot that is tied for more than one day is called somewhat permanent, and it is rabbinically forbidden to tie. The Tur and Mordechai<ref> As cited in Rema 317:1. Tur in 317 says that a knot is only permanent enough to be chayav chatas if it is able to be kept forever, but a knot meant to last seven days is patur aval assur.</ref> say that a knot that is tied for more than seven days is permanent. Rabbeinu Yerucham<ref> Cited in Beit Yosef 317:1</ref> says that only a knot that is tied for half a year or a year is considered permanent.