Kilayim Chapter 9, Mishnah 2. The Mishnah opens with two types of silk: one that resembles linen in appearance, and one that resembles wool.
"HaShiraim v'haKalach ein bahem mishum kilayim, aval asurim mipnei mar'is ha'ayin" - Shiraim and kalach are not subject to kilayim, but they are forbidden because of appearances:
"HaShiraim" - a type of silk that resembles linen or cloth.
"V'haKalach" - a type of silk that resembles wool.
Neither of them is subject to kilayim, since ultimately they are silk. Even though they may look like the materials that carry the prohibition of shatnez, they are not those materials themselves, only similar to them. Nevertheless, they are forbidden rabbinically because of appearances, and therefore one may not combine:
Shiraim, which resembles linen, with wool.
Kalach, which resembles wool, with linen.
Shiraim and kalach with each other: even though both are silk and there is no problem with them at all, since each one resembles a forbidden material, mixing them was forbidden because of appearances.
"HaKarim v'haKesatos ein bahem mishum kilayim, uvilvad shelo yiheyeh besaro nogei'a bahem" - Pillows and cushions are not subject to kilayim, provided that his flesh does not touch them:
Pillows and cushions do not carry the prohibition of kilayim, on condition that a person's flesh does not touch them. It is worth noting that the kesatos were not like our mattresses, but rather resembled very large pillows, on top of which people would sleep and lie down.
The Yerushalmi adds further conditions, three in all:
That his flesh not touch them, as the Mishnah states.
That they not be stuffed but empty, for if they are full, a person is liable to sink into them, and they would then be covering him to some degree.
That they be placed on a hard surface, so that when he sits or lies on them there is no possibility of the fabric rolling up and coming over him, as the verse says, "lo ya'aleh alecha" - it may not come upon you at all.
"Ein arai l'kilayim" - There is no such thing as temporary wearing when it comes to kilayim:
There is no category of temporary use with kilayim. Wearing shatnez is forbidden even in a temporary, incidental manner, and one cannot argue that since this is not permanent, defined, official wearing it is therefore permitted. Even fleeting wearing is forbidden.
"V'lo yilbash kilayim afilu al gabei asarah, afilu lignov es hamechess" - One may not wear kilayim even over ten other garments, even in order to evade the tax:
One does not wear kilayim even as a tenth layer over all the other garments, and even when the entire purpose of wearing it is to evade the tax. At times there were tax collectors who were not appointed by the government but were simply strongmen, who collected tax or collected more than the government authorized them to, and they would impose the tax according to the quantity of garments a person had in his possession. For this reason people would wear many garments in order to avoid the tax.
And even though his intention in this wearing is not for the sake of wearing at all, but only to conceal the garment, this opinion holds that there is still a problem with it, and the prohibition of shatnez remains in force.