The act of knotting was one of the necessary creative works (melacha) necessary for the building of the Mishkan (Tabernacle). During the making of the ten curtains that draped over the Mishkan roof, “when a thread tore during the weaving process, they [the Israelites] knotted the thread” (Talmud Shabbat 74b). As a melacha used in the Mishkan, the act of knotting is considered one of the 39 m’la’chot (creative acts) prohibited on Shabbat.
According to Jewish law, all knots are not created equal. When it comes to determining the permissibility of a certain knot on Shabbat, “Rabbi Judah stated a general rule: Any knot that is not permanent entails no culpability” (Talmud Shabbat 113a). An additional factor is whether it is a professional knot.
So what is the status of the three most common types of knot-tying in the modern age?
Neck Ties: In general, most people don’t intend the knot of their tie to be permanent. It is tied and untied each time the tie is worn. Knotting one’s necktie is therefore permitted by most authorities on Shabbat.
Garbage Bags and other plastic bags: Because it is common for people to tie these bags tightly, so that no garbage spills out, they require particular attention on Shabbat. A simple once over of two sides (similar to the first steps of tying a shoe) is permitted. A double knot, however, may not be.
Shoes: In general, shoes are tied temporarily and so a simple double loop knot is permitted. Double knotting one’s shoes, however, may be more of an issue. The purpose of the double knot is to make certain that the original single knot stays firm, which some might regard as creating permanency. However, the general intent of tying a double knot in a shoe is temporary. As there are different opinions regarding double knotting shoes, one should consult one’s personal rabbi.
It should be noted that any type of knot that may not be tied, may not be untied.
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