Sefer HaChinuch ספר החינוך

To not wear shaatnez: To not wear clothing that is composed of wool and flax (linen), and that is what is called, shaatnez. And about this is it stated (Deuteronomy 22:11), “You shall not wear shaatnez, wool and flax together.”

I wrote a hint about the roots of the commandment in the Order of Mishpatim about the matter of “do not keep a witch alive” (Sefer HaChinukh 62). And Rambam, may his memory be blessed, wrote about the explanation of the prohibition (in Sefer HaMitzvot, Negative Commandments 42 and in Guide for the Perplexed 3:37), [that] it is because the priests of idolatry would dress like this at that time. And he wrote further that this matter is still known with the priests that are in Egypt.

From the laws of this commandment is that which they, may their memory be blessed, said (Yevamot 5b, and see Mishneh Torah, Diverse Species 10:2) that there is only a prohibition from Torah writ of a combination of wool and flax if they are shua, tavui and noz. [This] is to say that the wool is shua by itself, meaning to say, beaten, and tavui (spun) by itself and noz by itself — and the explanation of noz is woven (see Rashi on Niddah 61b and Rabbenu Tam in Tosafot on Niddah 61b) — and so [too] is the flax shua, tavui and noz by itself; and afterwards he combined them together — for example, he wove them, or even if he tied them one to another, once he tied two knots, this is shaatnez from Torah writ, for which there is lashing. But the whole time that these three [processes] that we have written are not done to them, it is not shaatnez from Torah writ, but [only] rabbinically, according to the opinion of some commentators, and we do not administer lashes for it. But it is forbidden rabbinically if one of the [processes] that we have mentioned — either shua or tavui or noz — is done to them. And this is what they, may their memory be blessed, said (Mishnah Kilayim 9:9), “the ones alone are forbidden” — meaning to say, rabbinically. And there are some commentators that said that [even] with one of these [processes], it is forbidden from Torah writ — and when they said about this, “forbidden,” it is meaning to say from Torah writ.

And they, may their memory be blessed, said about this matter (Yoma 49a), “Even [if there are] ten beddings, one on top of the other, it is forbidden to [sit] upon them,” as we are concerned lest a fiber wrap itself upon his flesh. And they, may their memory be blessed, said this matter when the forbidden mixture underneath is soft, as then is there this concern of wrapping. (See Mishneh Torah, Diverse Species 5:12-13.) And this prohibition is rabbinic, as from Torah writ it is permissible, even when they are soft. And [it is] as they, may their memory be blessed, said, “‘It shall not come upon you’ (Leviticus 19:19) — but you can set it under you.” (See Mishneh Torah, Diverse Species 10:12-13.) And in Yerushalmi Kilayim 9:1, they said [that] pillows and comforters — even though they are hard — with full ones, it is forbidden to sit upon them rabbinically, since they double over on the one that is sitting upon them; but with empty ones ([this is what is written] in the Yerushalmi, [though there appears to be an error here in the Sefer HaChinukh, wherein it is written], the soft ones), since there is no concern that they will go on top, it is permissible to sit upon them, since (if) they are hard. And the explanation of the matter is [that it is] because the verse forbids shaatnez with an expression of wearing, meaning to say in the way of wearing is it forbidden and not in another way. And for this reason our teacher, God protect him, permitted us to put hats that are made from felt on our head to guard against the sun, since they are also very hard. And therefore, even though they have a forbidden mixture in them, they permitted them — as it is not the way of wearing something so hard. And there is one who wants to be stringent upon himself with this, and they did not protest against him. And for this reason — that it is the way of wearing that the Torah forbade — they also permitted the sellers of cloth to sell according to their way (to put the cloth on top of themselves) but only when they do not have any intention at all to warm up from it. And nonetheless, the modest and good wholesalers extend clothes from forbidden mixtures on a stick behind them so that they not touch them. And they, may their memory be blessed, said that there is no measurement to forbidden mixtures — that even one string in a large garment forbids all of it until he removes it. (See Mishneh Torah, Diverse Species 10:5.) Kelech — which is a type of wool and it grows on stones in the Dead Sea — is forbidden with flax rabbinically, because of the appearance [of sin] (marait ayin). And the rest of the details of the commandment are elucidated in Mishnah Kilayim 9 and in Tractate Shabbat and in the end of Makkot.

And this prohibition is practiced in every place and at all times by males and females. And one who transgresses it, and wears a Torah-level forbidden mixture — or covers himself with it — is liable for lashes. And if it is a rabbinic-level forbidden mixture, he is liable for lashes of rebellion. And one who wears a forbidden mixture — and even if he wears it the entire day — is only lashed one [set of lashes]. But one who takes his head out from the garment and puts it back in — even though he does not remove the entire garment — is obligated [lashes] for each and every [time that he does this]. And in what case does it apply that he is only obligated once for [wearing it] the whole day? When they gave him one warning. But if they warned him [several times] and they said to him, “Take it off,” and he [continues to] wear it for the time period [needed] in order to put it on and take it off — since they warned him, behold, he is obligated for each and every [such] time period that they warned him about it, and even though he did not take [it] off [and put it on anew].