Menachot 42 - Make Tzitzit on the Garment, not Attach a Garment to Tzitzit

The Torah says, "You will make yourself tzitzit on the corners of your garment." This means that the tzitzit is made when the garment is completed. If one made the tzitzit first, before the garment became obligated in tzitit, he would be wearing the tzitzit that was already made from before, not the one that he attached to the garment, and so it would be invalid. For example, if the garment tore, and he sewed it back with a woolen thread, then changed his mind and decided to use the thread as tzitzit, it would not be valid.

Can one unfasten the tzitzit on one garment and put it on another one? Rav says "no," but Shmuel says "yes," and this is one of the cases where Shmuel wins. We can even put the new tzitzit on the garment which already has the old ones, and this does not contradict the rule of "make but do not use already made ones" above, since in this case the removal of the old tzitzit is part of the making of the new ones.

Art: John Melhuish Strudwick - A Golden Thread