A bow knot is not considered a knot because one pull undoes the whole thing. Even if one bow is tied on top of another, it is not considered a knot because both bows can be undone with one yank (SA 317:5; MB ad loc. 29). A single knot is not considered a knot either, since it does not last. Since bow knots and single knots are not considered knots, one may tie them even if one intends to keep them intact for a long time.
Some maintain that a single knot with a bow on top of it (which is how many people tie their shoes) retains the status of a single knot. Accordingly, it is not considered a knot, and may be tied without any worries. Others are stringent and maintain that since the two knots together are stronger than one, it should be considered a regular “layman’s knot.” Accordingly, one may tie it if one intends that it last for less than a week. However, if it is meant to last for a week or more, one may not tie it. It is appropriate to follow this position le-khatĥila. However, one may tie a gartel around a Torah scroll using a single knot with a bow on top of it, even if it is intended to remain that way for many months.
A single knot followed by a bow followed by a single knot, which people use when they wish to tie their shoes more tightly, is considered a regular knot. If it is meant to last less than a week, one may do so; if it is meant for a week or more, one may not do so. Some are meticulous and completely avoid tying such knots on Shabbat.
If one ties and unties his necktie each time he wears it during the week, he may tie it on Shabbat as well. If one ties his necktie and leaves the knot intact for an extended period of time, he may not tie it on Shabbat. In a case of necessity, he may be lenient and tie it on Shabbat, as long as he intends to undo the knot on Saturday night.
. According to Agur, Rema 317:5, Levush, and the Vilna Gaon, a knot with a bow on top is not considered a knot at all, and may be tied even for an extended period of time. According to Mordechai, Taz, and MA, it is considered a layman’s knot, which, according to Rashi and Rosh, should not be tied for an intermediate amount of time. Many take this position into account and write that one may not leave one’s shoes tied for more than one day (MB 317:29; SSK 15:56). But it would seem that this is excessively stringent. In practice, one may tie such a knot with the intent to leave it intact for up to a week, because this is a case of a rabbinic rule with multiple doubts: a) If the halakha follows Rif and Rambam, since this is a layman’s knot that is not meant to be permanent, there is no prohibition according to the majority of poskim; b) according to Rema, it is not considered a knot at all, and may be tied even for an extended period of time; c) according to Mordechai and Tur, up to a week is considered short term. Therefore, even Rashi and Rosh would agree that there is no prohibition on tying this knot (and BHL 317:4 s.v. “she-einam” agrees that at a time of need one can be lenient). Even for those who are stringent, it is only a rabbinic prohibition. Therefore it is sufficient that we are stringent in avoiding tying a knot with a bow on top for more than a week.Regarding a Torah scroll, the custom is to be lenient and tie a knot with a bow on top, even for an intermediate amount of time. While some poskim are stringent even in this case if the knot is meant to last more than a day or a week (Minĥat Shabbat 80:155; SSK 15:56; Brit Olam, Ha-kosheir U-matir §4; Orĥot Shabbat 10:28), nevertheless, the lenient position is correct because this is another case of a rabbinic rule with multiple doubts: a) For those who follow Rif and Rambam here, there is no prohibition at all; b) even according to Rashi and Rosh, there is a disagreement whether there is a prohibition, and Rema maintains that this is not considered a knot at all; c) even according to those who are stringent, the prohibition is rabbinic. Rif, Rambam, and Tur rule that for the sake of a mitzva, the Sages permitted tying a knot that is rabbinically prohibited (SA 317:1; MB ad loc. 13). This is also the position of Tzitz Eliezer 7:29.
Regarding a single knot followed by a bow followed by a single knot, the rule is the same as for a regular knot. Some are inclined to be stringent and consider it the equivalent of a double knot, which one may not tie even for a single day. However, many are lenient even regarding a double knot, and there is even more reason to be lenient here, where one good yank can undo the whole thing. Therefore, it should be considered a regular knot, and it may be tied for less than a week (see SSK 10:14-15 and Harĥavot 13:13:5).
. It is impossible to transgress a Torah prohibition with a necktie, because it certainly is not a craftsman’s knot and it is not meant to last forever. However, it would seem that if a necktie is tied for an intermediate amount of time, one transgresses a rabbinic prohibition according to Rashi and Rosh. Even though one yank can undo it, since it looks like a knot and can last for an extended period of time, it is considered a type of knot. However, be-di’avad it would seem that even one who usually ties his necktie for an intermediate amount of time may be lenient, as long as he plans to untie it on Saturday night. According to Rif and Rambam, since it is not a craftsman’s knot and is not intended to last forever, it is not prohibited at all. Even according to Rashi and Rosh, it may be that since one intends to undo it on Saturday night (as many people always do anyway) this is considered short term and is not forbidden (see SSK 15:62).