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Strings that unraveled: 
If any of the eight strings unraveled from its spinning after it was tied to the garment, the unraveled area is considered cut off from the Tzitzis and it hence does not enter into the minimum measurement of length required for a torn string, as explained in Halacha 5. Thus, if an entire string unraveled and split to two strings, we view it as if the entire string has been cut off. If only part of the string unraveled while the remainder did not unravel then we view the unraveled area as if it has been cut off.
If three strings unraveled: If three strings unraveled to the point that there does not remain 24 cm of raveled string on any of the three strings, the unraveled area is viewed as if it were cut off and the Tzitzis is thus invalid, as explained in Halacha 12D.
May one make knots at the end of the strings to prevent them from unraveling? See Halacha 11 that some have the custom not to make knots at the end of the strings in order not to add to the amount of knots made on each fringe. However, one may dip the tip of the strings into glue and the like.
What is the status of the string if only some of its threads unraveled?
- Example: Today we are particular to intertwine 8 threads for every string. What is the law if 2 threads unraveled and 6 remained intertwined?
The Tzitzis remains Kosher. It is only considered unraveled if all 8 threads are no longer intertwined. Some Poskim however question if it is valid if 5 of the 8 threads have unraveled.
What is the law if a string unraveled in the area of the Gdil/braid, or by the hole of the corner?
The Tzitzis is invalid.
May one ravel the string back up in order to validate an unraveled string?
If the Tzitzis is already invalid due to the unraveled string, then it does not help to ravel it back up unless one unties the entire Tzitzis and reties it afterwards. If however the Tzitzis still remains valid despite the unraveled string, then one may re-ravel it in order to lengthen the string.
 12/7; Siddur Admur; See Michaber 11/3; Kitzur SHU”A; Olas Tamid 11/3
 Certainly if this occurred prior to it being tied to the garment the unraveled area is invalid. [Elya Raba 11/3; Olas Tamid brought in Kaf Hachaim 11/12] Perhaps this was emphasized by Admur in 12/7 because there he is lenient regarding if three strings cut off. Vetzaruch Iyun.
 The reason: As the Tzitzis are invalid if the strings are not spun and intertwined, as explained in Halacha ?! [Admur 12/7]
 Siddur Admur [as simply understood-see below]; Piskeiy Teshuvos 11/13 based on Admur
Ruling of Admur in Shulchan Aruch and other opinions: The Shulchan Aruch [Admur 12/7; Michaber 11/3 as explained in M”A 11/4] rules that in the above case one may be lenient to validate the Tzitzis. The reason for this is because there are opinions that validate a Tzitzis even if it has three torn strings, so long as each of the four long strings maintains Kdei Aniva. [see Halacha 8D in footnotes] Now, although we do not rule like this opinion, nevertheless since there are opinions [Rambam 1/10] that hold that the Tzitzis strings do not need to be intertwined at all, therefore regarding the unraveling of a string one may be more lenient than if the string has completely tore, and one may thus rely on the above opinion that validates three torn strings. [Admur ibid; M”A ibid; M”B 11/17] In summary, in the Shulchan Aruch the Michaber and Admur hold that regarding the unraveling of a string, one may be lenient to consider the string still intact if it contains Kdei Aniva of intact string.
Ruling of Admur in Siddur: In the Siddur, Admur rules immediately after stating that three cut strings are invalid, that unraveled strings have the same law as cut strings, hence implying that in the Siddur he retracted his ruling and is stringent in this matter. [Piskeiy Teshuvos ibid; See Siddur Raskin footnote 119] However see Ketzos Hashulchan 7 footnote 55 which questions whether the intent of Admur in the Siddur is to completely invalidate the Tzitzis, or simply to invalidate wearing it with a blessing. See also Nesiv Hachaim 7/2 that makes no gloss of Admur’s ruling in the Siddur. See Rav Raskin ibid
 M”B 11/17; See Ashel Avraham Butchach 11; Aruch Hashulchan 11/8; Chazon Ish 3/16 regarding why there is no issue here of Baal Tosif.
 Tehila Ledavid 12/4
 Implication of Admur ibid; Chazon Ish 3/16; Kerem Shlomo; Toras Chaim Sofer 11/8
Other opinions: Some Poskim rule the Tzitzis remains valid. [Tehila Ledavid 11/10; Biur Halacha 11 “Venaasu” leaves this matter in question]
 Biur Halacha 11 “Hu”; Poskim in Piskeiy Teshuvos 11/13 footnote 81
Other Poskim: Some Poskim rule in such a case one may be lenient to ravel the string even if the Tzitzis is now invalid, being the Rambam does not require stings to be intertwined even initially, and one may rely on his opinion in a time of need. [Aruch Hashulchan 11/8]