Mishnah Berurah — משנה ברורה

Enough to tie a bow - And even though there are those who hold that it is invalid [in a case] where all [the strings] were cut, even if there was left over enough [on each string] to tie a bow, as is written in Siman 12... nevertheless, by interweaving (ie. in a case where the interweaving unraveled) we rely on the first opinion, for the law is in accordance with him, as he (ie. the Shulchan Aruch) concluded there (in Siman 12) [see in Magen Avraham and Pri Megadim]. And you should also know that the Shulchan Aruch is referring specifically to where each string was interwoven into two (ie. after the raw wool is spun into threads -called "t'viah" - each thread is then doubled over and spun/interwoven -called "sh'zira" - to make a thicker string). But if the string was doubled over [again] and interwoven into four or [doubled over again after that to make] eight, as is found nowadays, and then it unraveled into sixteen (ie. each of the 8 ציצית strings unraveled and you now have 16 strings), [then] even if there's not [a section of string] left over [that's long enough] to tie a bow, it is valid. Because at the very least, each string is still interwoven into two. And regarding [the question of whether this is a transgression of] "do not add", see further in Biur Halacha.