[A garment made of] wool of ewes or rams, mixed with linen is forbidden, for it is kelayim (shaatnez). Whether a woolen garment was sewn to a linen garment even with silk or hemp thread, or whether a woolen garment was sewn with linen thread or vice versa, whether linen thread was tied with woolen thread or braided together, all of these are forbidden for they are shaatnez. Sewing one stitch and tying it, or sewing two stitches even without tying them is enough of a connection to make it shaatnez. Therefore, it is forbidden to join a woolen garment with a linen garment even with a needle [pin] without thread.

Sheepskins from which clothing is made, may be sewn with linen threads, and we are not concerned with the woolen hairs although they are joined with linen thread, because these woolen hairs are not considered threads and are of no use and are considered non-existent.

To join wool and linen (flax) with something between them, for example, to take a piece of leather and sew or fasten a piece of wool on one side and a piece of linen on the other side, is forbidden by Scriptural law, according to the Rambam; but some Poskim permit it.¹. Therefore, according to the latter opinion, skins sewed together with linen threads may be used as a lining for a woolen garment. (And though it is possible that the hemp thread, with which the skins are sewn to line the woolen garment, will penetrate among the flax threads with which the skin were sewn, we are not concerned about it). And this is the prevailing custom.² 299:2. Nevertheless, one who is spiritially sensitive, should be strict and heed the opinion of the Rambam.

footnotes: ¹ Rosh ² Ramah

Even if ten mats [lie] one on top of the other, and the bottom one is shaatnez, it is forbidden to sit on the top one.³ is a Rabbinic prohibition, and is only forbidden if the mat is soft, so that when you sit on it, it is possible that at least one thread will fold over under you. (See Shulchan Aruch 301: 1)

footnotes: ³ Sitting on Shaatnez

If a large garment contains shaatnez at one end, it is forbidden to cover yourself even with the other end, even if the shaatnez part is resting on the ground.

If you sew a garment of shaatnez for a gentile, you may sew it in the regular manner, even though the garment rests upon your knees, provided you do not have intention to derive pleasure from its resting on you. Also clothing dealers, who carry [coats] on their shoulders to sell them, are permitted (to carry them) provided they do not intend to use them for protection from the cold or the rain. Nevertheless, God-fearing people carry them on a stick.⁴, it is nevertheless permitted to try it on for size etc. If it definitely has Shaatnez, you must consult a Rav.

footnotes: ⁴ If there is a doubt whether the garment contains Shaatnez

Handkerchiefs, and bath towels, tablecloths and similar things, also the cover of the lectern in the synagogue on which the Torah is read, are subject to the law of shaatnez. It is also forbidden to have curtains containing shaatnez, but the curtain covering the Holy Ark may be made of shaatnez.

It is permitted to ride in covered wagons, (coaches) which are upholstered on the inside, with woolen cloth containing shaatnez, providing you are careful not to lean on the sides that contain shaatnez. You must be especially careful not to sit on cushions containing shaatnez. Some Poskim even permit to sit on such cushions, since they are made in such a way that they do not bend over the sides of the person.