Halakhah do Kilajim 9:1

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 if 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.