In the previous Halachot we have discussed some basic laws of tying and untying knots on Shabbat. The general rule is any knot that is either “professional,” i.e. requires some skill to make, or “permanent,” i.e. is meant to last for a prolonged amount of time, is forbidden to be made on Shabbat.
Tying an Additional Knot on Top of a Knot Which Existed Before the Onset of Shabbat
We have already mentioned that some authorities rule stringently and prohibit tying a double knot on Shabbat, for this is considered a professional knot. (For instance, when one ties one’s shoes, one usually makes one knot and another bow on top of that. Making another knot on top of the first one makes the knot stronger.)
Thus, the Poskim write that if there was one knot tied before Shabbat, one should not make an additional knot on top of it, for the second knot is the primary knot forbidden on Shabbat. This is similar to a situation where one letter was written before Shabbat and writes another letter on Shabbat thereby completing the word. For instance, if the letter “Alef” was already written and one then adds the letter “Bet” on Shabbat, it is considered that one has written the word “Av” (“father” in Hebrew), a word with a meaning, on Shabbat, which is a Torah prohibition. (It is a rabbinic prohibition to write even one letter on Shabbat.)
Similarly, if one adds another knot to a pre-existing knot on Shabbat, one transgresses the forbidden work of tying on Shabbat. Maran Rabbeinu Ovadia Yosef zt”l rules likewise in his Chazon Ovadia-Shabbat, Part 5, page 68. Although we have written previously that Maran zt”l rules that one who ties a double knot on Shabbat has on whom to rely, it is nevertheless worthy to act stringently regarding this matter.
Tightening Loosened Tzitzit
If one’s Tzitzit loosened on Shabbat, one may not tighten it on Shabbat, for this resembles tying a double not. This is especially true since there is room to say that tying Tzitzit is a kind of “professional” knot and Tzitzit are considered tied “permanently” since one never intends to untie them.
Plastic Cable Ties
The modern-day Poskim discuss whether or not one may use plastic cable ties on Shabbat. Should their use pose a concern of the forbidden works of tying or building on Shabbat? Maran zt”l rules (ibid. page 75) that there is no prohibition involved and one may use them on Shabbat, for this is not considered a “knot”; rather, the plastic cable is only caught and tightened. The Torah never discussed such a reality and our Sages never banned this kind of action, for this does not resemble the forbidden work of tying at all.
Summary: One may not tighten loosened Tzitzit strings on Shabbat. One may use plastic cable ties on Shabbat.