One may not tie a double knot even if he plans to untie the knot on the same day. Since this is a strong knot that can remain intact for an extended period, it may be that it is comparable to the knot of a craftsman (Shiltei Giborim). According to some Rishonim (Rif and Rambam), tying such a knot for the short term is rabbinically forbidden. All this refers to a tight double knot, which one might use to tie his shoes or to close a garbage bag. But a woman may tie a head scarf this way, since the scarf is not pulled tight. Those who are especially meticulous are stringent and do not use a double knot to tie their head scarves.
If it happens that one’s shoes were tied with a double knot and this causes him anguish, he may undo the knot.
One may not tie off the end of a thread even with a single knot, the way one does when sewing or when tying the end of a tzitzit string. Since this knot is strong, it might be considered a craftsman’s knot. According to some poskim, if it is meant to be temporary it is rabbinically prohibited, while if it is meant to be permanent it is prohibited by Torah law (Smag; Rema 317:1). Similarly, one may not use a tight knot to tie a plastic bag with food in it, but one may use a bow knot or tie the bag handles together with one tie.
. According to Rashi and Rosh, any knot tied for a short period of time may be tied and untied. Accordingly, even a double knot that is generally tied for the short term may be untied. However, according to Rif and Rambam, who maintain that it is rabbinically prohibited to tie a craftsman’s knot for a short period of time, there is disagreement regarding a double knot. Shiltei Giborim states that we must take into account the possibility that a craftsman’s knot is defined by its strength; since a double knot is a strong knot, it should be prohibited as well. This is the position of Rema 317:1; Pri Ĥadash; Rav Pe’alim, OĤ 2:44. In contrast, AHS 317:3 and Ĥazon Ish 52:17 maintain that this constitutes a stringency, as a craftsman’s knot requires professionalism, while a double knot does not. Since many poskim who follow Rif and Rambam forbid a double knot, le-khatĥila one should avoid it. However, at a time of need, if one’s shoes were tied with a double knot he may undo them. According to Rashi and Rosh, one may do so because they were tied for only a short time; even according to Rif and Rambam, some are permissive. Furthermore, even according to those who prohibit this, the prohibition is rabbinic, so at a time of need one may rely on those who are lenient. All of this applies to a tight double knot, but untying the knot of a head scarf that is not pulled tight is not prohibited. (This is the approach of SSK ch. 15 n. 175; Menuĥat Ahava 3:14:5; see Orĥot Shabbat ch. 10 n. 16. However, Rav Pe’alimloc. cit. and Kitzur SA 80:45 are stringent.)