Commentary for Menachot 80:19

But doesn’t Rabbi Zeira say, "If one affixed [tsitsit] to a garment that already had them affixed, it is fit": It is a wonder: How might you think that Rabbi Zeira does not hold of, "Prepare [it], and not from what has already been prepared." Behold, Rabbi Zeira himself holds from it above! And it can be said (the answer is) that you might have thought that since the beginning of its placement was for the sake of the commandment, "Prepare it, and not from what has already been prepared," does not apply. But above, when it was cut, the [status of the] placement was nullified.

It is involved with [violating] "do not add"; it is not considered an action: As since he transgressed a prohibition, it is not considered an action; so [only] at the time of the cutting of the first [strings], is it the making [of the second strings]. So it is not similar to that [case] in the first chapter of Sukkah 11a, in which we do not say their cutting is their making. And this requires [further] investigation.

Sky-blue strings are not subject to diverse kinds, and even on a cloak that is exempt: From its saying, "and even," and not saying, "even" – without "and" – it is implied that [also] when it is not [a case of] a cloak that is exempt was it necessary to inform us that there is no [prohibition] on account of diverse kinds. And in this manner did we make an inference in the Chapter [entitled] Taraf Hakalpi (Yoma 46a): Rabbi Meir would say, "[Regarding] limbs of a burnt-offering from a weekday that remained [on the altar and were not fully consumed], one should make an arrangement of wood for them and arrange them [upon it], and even on Shabbat." And the discussion from there shows that we are informed of two things [from Rabbi Meir's statement]. And Rabbenu Tam says that here too are we informed that even at night is there no [prohibition] on account of diverse kinds, even though it is not the time of [the obligation of] tsitsit. For the verse completely permitted diverse kinds within tsitsit. And that which we decreed earlier [regarding diverse kinds] on account of a cloak for the night, is only [regarding] a cloak that is designed specifically for the night. (And Rabbenu Tam contradicts [his] explanation above [in the second entry on this page 40b]:) As a cloak for the day is obligated [to have tsitsit] even at night; and so is it in the first chapter of Kiddushin 34a. And you should know that we say later (Menachot 44a), "[In the case of] a borrowed cloak, it is exempt from tsitsit for thirty days; from then on, it is obligated" – and even [if the tsitsit are] from diverse kinds, even though it is exempt from the Torah (i.e. the obligation is only rabbinic and therefore would not have been able to override a Torah-level prohibition if one existed here). For one can not differentiate, once there is no [prohibition of] diverse kinds for the man to whom it belongs. And so too for women [wearing diverse kinds would be permitted in a cloak (designed) for the day, even though they are exempt from tsitsit]. And that which we say (below, Menachot 43a) concerning the priests (kohanim), "True, [wearing priestly garments that contain diverse kinds] is permitted at the time of the [Temple] service; when not at the time of the service, it is not" - is not similar at all [to the case of tsitsit]. For it is written (Deuteronomy 22:11-12), "Do not wear shatnez (diverse kinds) [...]. Make tassels for yourself" – to say in the case of tassels (tsitsit), you may wear shatnez. [Hence] it is implied that the Torah permitted all [such] wearing. Moreover with a cloak made for his [own] benefit, the verse obligated diverse kinds, whereas [the priest] only [has an obligation to] wear priestly garments at the time of the service. But Rabbenu Tam also said that priestly garments are also permitted not during the time of the service; and this is what was said [in Menachot 43a]: "True, [wearing priestly garments] is permitted at the time of the service," meaning those designed for the service; "when not at the time of the service," meaning those not designed for the service, "it is not permitted." And so is it shown in the Tosefta in Kilaim (Tosefta Kilaim 5:14), as it is learned, "[Regarding] priestly garments and the garments of the high priest, there is no [prohibition] on account of diverse kinds. [In a case of] the garments of a high priest that went into the province [wearing his garments], he is liable; within the Temple, he is exempt - whether to serve or not to serve." And it requires [further] investigation whether night is similar to the province, which is liable. And it can be said that that he is liable [only] rabbinically. And [regarding] that story of Simon the Just in the Chapter [entitled] Ba Lo (Yoma 69a), of "a time to do for the Lord, rescind Your Torah" – rescinding is also applicable with rabbinic [laws]. Nevertheless, it is not implied that way. For it rather looks more likely that he is liable on the Torah level, and one should not compare places [at night] to the province - as there is no dispensation in the province. But you should know that it does not distinguish there between a time that is fitting to serve and a time that is not fitting to serve. And that which we say [in] the Chapter [entitled] Ba Lo (Yoma 69a) concerning [the teaching about] priestly garments that "they fold them and place them under their heads" is not actually under their heads, but rather next to their heads on account of diverse kinds - is about spreading out bedding, which is not similar to wearing which the Torah permits. But regarding tsitsit, there is no distinction between spreading bedding and wearing. And we learned in Sifrei, "A cloak designed [both] for day and for night is obligated in tsitsit. Rabbi Yehudah ben Bava says, 'The Sages specifically negated a woman's garment from [the obligation of] tsitsit, and they obligated her cloak, because her husband covers himself with it." Therefore, it appears that diverse kinds are permitted in tsitsit, whether during the day or during the night, or with a woman.