שולחן ערוך · אורח חיים

The things that render the tsitsit pasoul. This siman has 3 seifim:

Seif 1: if all the threads of the corner were cut but there remains enough to make a loop (כדי עניבה) with all the cut threads together — kosher; and if there does not remain enough for a loop, even for a single thread cut entirely — pasoul. Since each one is folded in two, if two heads were cut — pasoul, for perhaps a whole thread was cut (שמא נפסק חוט אחד). And per our custom to mark with a סימן the 4 heads so that these 4 heads are always on one side of the knot — if two heads were cut on one side, kosher, for they are surely two [distinct] threads. [Glose: for ר״ת one validates only if 2 whole threads remained... yet where possible it is good to heed the view of Rabbeinu Tam; and the custom follows ר״ת.]

Seif 2: where כדי עניבה is required, and since the threads are thick one cannot tie them, whereas were they thin they would have enough for a loop — kosher. [Glose: and one measures with medium threads (בינונים).]

Seif 3: כדי עניבה — for Rachi from the ענף (the hanging, untressed part), and for R"i, even if the whole ענף was cut and enough for a loop remains only from the גדיל (the tressed part) — kosher; the custom of the world follows Rachi, and where it is not possible one may rely on R"i.