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Many times, for those who have two double knots with eight strings in each knot (i.e. not following the opinion of the Rambam), the knot starts to come loose often, sometimes even several times a day. If it is not tightened, occasionally it will completely come out by the end of the day.

My question is, if someone notices on Shabbos that their tzitzis knot is coming loose, and is slipping down the strings, can they slide it up closer to the top, without tightening it, so that it won't come completely undone and begin to unravel further?

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This ehow explains how to tie tzitzis. At the end it says, "Tips & Warnings Wet all of the knots with boiling water then let them air dry. This will make the knots dry tighter and stronger to prevent unwinding." The threads shrink as they dry and this is what tightens the knots. I am unsure as to whether boiling water is essential or lower temperatures are also effective. – Avrohom Yitzchok Aug 12 '14 at 20:19
If your double knots are frequently slipping open, they might be granny knots rather than reef knots. Untying the second knot, switching which set of strings goes over which, and retying may solve your problem. – Isaac Moses Aug 12 '14 at 20:20
@IsaacMoses, just not on שבת. – Seth J Aug 12 '14 at 20:21
@SethJ, indeed. The fix I suggest should be done on one of the other six days of the week, to alleviate this problem happening on Shabbat. – Isaac Moses Aug 12 '14 at 20:22

The book The 39 Avoth Melacha of Shabbath* on Page 32 has an illustration that specifically forbids this (as Rabbinic, IIRC a Torah prohibition).

Image from book

It sources the statement to Kitzur Hilchos Shabbos 25, 2 and Shmiras Shabbos Kehilchasa 15, 50.

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I think the illustration is of a knot about to be tightened. The Shmiras Shabbos reference is referring to tightening a knot, not to just sliding it closer without tightening it. – Y ez Nov 5 '14 at 21:19

If the knot is high up to the top and if it is about to be undone you can bring down lower but do not tighten it. Shulchan aruch harav hilchos tzizis

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Can you give edition, chapter and paragraph numbers? – Double AA Aug 13 '14 at 0:04
i will try to find – Dovid Benizri Aug 13 '14 at 0:21
@DovidBenizri any luck finding this? – Y ez Nov 5 '14 at 21:15

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