The Sages taught in a baraita: With regard to ritual fringes on a linen cloak, Beit Shammai deem the cloak exempt from ritual fringes due to the fact that the sky-blue strings must be made from wool, and there is a Torah prohibition against wearing a mixture of wool and linen. And Beit Hillel deem a linen cloak obligated in the mitzva of ritual fringes. And the halakha is in accordance with the statement of Beit Hillel.

Rabbi Eliezer ben Rabbi Tzadok says: But is it not the case that anyone who affixes sky-blue strings to a linen cloak in Jerusalem is considered nothing other than one of those who causes others to be astonished at their behavior, as it appears that he is violating the prohibition against wearing a garment containing wool and linen?

The baraita concludes: Rabbi Yehuda HaNasi says: If so, that the halakha is in accordance with Beit Hillel and a linen cloak is required to have ritual fringes, why did the Sages prohibit attaching ritual fringes to linen garments in Jerusalem? It is because people are not well versed in the halakha and might ultimately wear garments of wool and linen even when it is not necessary for the mitzva of ritual fringes.

Rava bar Rav Ḥana said to Rava: If that is the concern, then let ten people take linen cloaks with ritual fringes and go out to the marketplace and thereby publicize the matter, i.e., that it is permitted to affix wool strings to a linen garment due to the mitzva. Rava answered: All the more so people would be astonished at us for acting in such an unconventional manner.

The Gemara suggests: Let the Rabbis teach during their public lecture that affixing wool strings to a linen garment is permitted for the mitzva of ritual fringes. The Gemara answers: Wearing strings on a linen garment is prohibited because of a rabbinic decree due to the concern that people might use strings that were dyed blue with indigo [kala ilan], instead of with tekhelet, the sky-blue dye produced from the ḥilazon (see 44b), in which case they would not fulfill the mitzva of ritual fringes and would violate the prohibition against wearing garments containing wool and linen.

The Gemara suggests: Even if one’s blue strings are not dyed with tekhelet as required for the mitzva, let them be considered merely as white strings. In the absence of tekhelet one fulfills the mitzva with white strings, and therefore it should be permitted to affix white woolen strings to a linen garment. The Gemara explains: Since it is possible to affix white strings that are the same type of material as the garment, i.e., linen, and thereby fulfill the mitzva without overriding the prohibition against wearing a garment made from wool and linen, one may not affix white wool strings to a linen garment.

The Gemara notes: This is in accordance with the opinion of Reish Lakish. As Reish Lakish says: Any place where you find a positive mitzva and a prohibition that clash with one another, if you are able to fulfill both of them, that is preferable; and if that is not possible, the positive mitzva shall come and override the prohibition. In this case, the clash is between the mitzva of ritual fringes and the prohibition against wearing a garment that contains wool and linen. One can fulfill both of them by using white strings that are linen instead of wool if the garment is made from linen.

The Gemara suggests: And let them test the strings to ascertain whether they are dyed with indigo or with tekhelet, as explained by the Gemara (42b). The Gemara responds: Rather, the prohibition against affixing ritual fringes to a linen garment is a rabbinic decree due to the concern that perhaps the sky-blue strings were colored with tekhelet dye that had been used for testing the color of the dye in the vat and therefore became unfit (see 42b). In such a case, there would be no fulfillment of the mitzva of sky-blue ritual fringes to override the prohibition against wearing a garment of wool and linen.

The Gemara suggests: Let the Sages write letters informing dye producers that tekhelet that was used for testing the color of the dye in the vat is unfit for ritual fringes. The Gemara explains: Shall we go and rely on letters, assuming that dye producers will follow the instructions they contain? Rava responded to this and said: Now,