Mishnah Berurah — משנה ברורה

Two ends etc. - And the same applies if all the ends tore from one side, and from the four ends on the second side there is only left on each one enough to make a loop [around the other strings], it would also be valid, because it is only invalid, according to this opinion, if the same string tore on both ends. And [regarding] what the Mechaber (ie. Shulchan Aruch) wrote - "that behold, there is [left] on the second end more than [what's needed] to make a loop [around the other strings]" - this is not an exact thing (ie. when he says there is "more than"). A question: ציצית strings that tore and he went and tied them [back together], if it's effective - the fact that they were made whole through his tying. The later rabbinic authorities concluded that the law is like this: if from the outset, before he made the ציצית from the strings, they tore and he tied them with a permanent knot, and afterwards made them into ציצית, it is valid because the knot was a total attachment (ie. tying a torn string back together with a permanent knot is considered one string now). And similarly, [if it tore] after [the ציצית] were made in a valid way, meaning if one end tore after the ציצית were affixed to the garment, even if there isn't left enough to make a loop [around the other strings], tying it now works. And even if the second end of that same string tears afterwards, it would be valid, since when he tied it, the ציצית were valid. But if the strings became shortened in a way that invalidates the ציצית because of that, for example: if both ends tore and there wasn't left enough to make a loop [around the other strings], or if the string tore by the hole, it doesn't help to retie it for that falls under the category of "You shall make it, but not from that which is already made" though an invalidation. And certainly if the string was shorter than it's [required] length from the outset, at the time of making the ציצית, for sure it doesn't help to go back and tie it (ie. to tie an additional length of string to the short one) after the windings and the knot of the first section [of the ציצית], for that is literally "You shall make it, but not from that which is already made". And this is as was seen previously in Siman 11, Se'if 13 regarding cutting the ends of the strings.