45) Usually when we want to tie something with two strings or ropes, or when we wind around a string or rope, and tie the two ends together, like a belt, we tie a double-knot because a single knot will not hold. It is forbidden on Shabbos to tie a double-knot, even on something that is usually untied the same day. Therefore, you must be careful when wrapping a scarf around your neck, not to make a double-knot on Shabbos. Similarly on Friday, Erev Shabbos, you should not make a double-knot, for if you do, it will be forbidden to untie it on Shabbos, as discussed below. Similarly, to make even one knot at one end of a string or rope, or to make one knot on both of them is forbidden, since with this method even a single-knot holds strongly. To take the two ends together (i.e. in a permissible way), tie them with a single-knot and a loop on top (Ed. Like the way we tie our shoes), if it is something that is usually untied the same day (Footnote #152: According to the Mishnah Berura, something that is tied Friday night and will be untied on Shabbos during the day is also permitted because if it is less than 24 hours, it is considered the same day, and it is permitted.) If not (i.e. not usually untied the same day), it is forbidden, even if you intend to untie it the same day. But making two loops, one on top of the other, is permitted, and it is permitted to make even many loops, even if it is intended that they remain for many days. 46) A knot which is forbidden to be tied on Shabbos is also forbidden to be untied on Shabbos (e.g. if it was tied before Shabbos.) If it causes discomfort it may be untied by a non-Jew.
Note: According to the view of some authorities, there are additional leniencies for untying knots in difficult situations. We quote from The 39 Melachos, by Rabbi Dovid Ribiat, vol. III, pp. 808-809: Rabbinically-prohibited knots may be undone to eliminate physical discomfort. Examples: 1) A double-knot in a shoelace or other garment may be untied if the knot prevents removal or use of the shoe (causing considerable hardship), because tying this knot is not a Torah-prohibition. 2) If, while tying shoes, a tight double-knot was accidentally made (instead of the intended bow-knot), untying the double-knot is permitted.