By Torah law, one may sit or lie on sha’atnez fabric, even though it provides warmth. The Torah prohibition applies only when the sha’atnez is above or alongside the body in the manner of clothing, not when it is beneath the body, as the verse states: “A garment of mixed kinds, sha’atnez, shall not come upon you” (Vayikra 19:19). The Sages, however, prohibited sitting or lying on sha’atnez fabric, lest a loose thread wind around one’s skin or one mistakenly use the fabric as a covering. To prevent such errors, they extended the prohibition even when the sha’atnez is covered by ten layers of permitted fabric (Beitza 14b; SA YD 301:1).
If the sha’atnez fabric is too stiff to be used as clothing and lies on a firm floor or chair into which the person does not sink, however, the Sages did not prohibit sitting or lying on it. A layer must separate the fabric from the body, so that any loose thread cannot wind around the skin (SA YD 301:1). One may therefore lie on a stiff sha’atnez rug when another fabric is spread over it, or sit on a chair upholstered with sha’atnez when pants or a dress separate the body from the chair. Even if the upholstery itself is soft, it is treated as stiff fabric as long as it is stretched taut over the chair with nails or glue (Panim Me’irot 2:113).[2]
[2]. The Sages prohibited sitting or lying on sha’atnez fabric, lest a loose thread wind around one’s skin. They extended this decree even to a case in which ten layers of bedding separate the person from the sha’atnez (Beitza 14b). Some maintain that this extension applies only to fabric commonly used for clothing. If the fabric is not ordinarily worn, one may lie on it as long as it is covered (Rid; Riaz; Ran; Maharshal). Most authorities, however, maintain that the decree applies even to fabric not ordinarily used for clothing (Rashi; Rambam; Meiri; Sefer ha-Ḥinukh; Rosh; SA YD 301:1; Levush; Baḥ).
If the sha’atnez fabric is stiff, however, one may sit on it (Beitza 14b). According to most authorities, it must also be covered, so that no loose thread can reach the skin and wind around it (Rambam; R. Shimshon of Sens; SA YD 301:1; Shakh YD 301:4; and others). Some permit sitting on stiff fabric even without an intervening layer (Rid; Riaz; Ritva), while others permit this only when the fabric is extremely stiff, like a donkey’s saddlecloth (Rosh; R. Yeruḥam).
A further rule applies to sha’atnez cushions and pillows. The Mishna permits sitting on them “provided one’s flesh does not touch them” (Mishna, Kilayim 9:2). The Yerushalmi explains that this applies only when they are empty and spread flat over a hard surface. If they are stuffed or placed on a soft surface, so that they bend upward around the person sitting on them, one may not sit on them even with an intervening layer (Yerushalmi, Kilayim 9:2; SA YD 301:1). Many explain that this is another application of the prohibition against lying on soft sha’atnez even beneath ten layers of bedding: when the cushions can bend upward, a loose thread may once again wind around the body. Others maintain that the cushions’ bending around the body is itself rabbinically prohibited because it is considered placing sha’atnez upon oneself. Ginat Veradim maintains that this is prohibited by Torah law, as does R. Shlomo Kluger (Tuv Ta’am Va-da’at, 3rd series 2:121). It seems to me that this prohibition applies only when the person sinks into the cushion deeply enough that the sha’atnez fabric at the sides becomes more vertical than horizontal, at which point it can be considered a form of wearing.
Soft sha’atnez upholstery that is stretched tightly over a chair, so that it cannot bend upward around the person sitting on it, is treated like a stiff surface. According to most authorities, one may sit on it as long as clothing or another layer separates it from the skin (Panim Me’irot 2:113; PT YD 301:1; AHS YD 301:9).
The Gemara indicates that a sha’atnez curtain is prohibited because a servant might occasionally wrap himself in it for warmth (Beitza 14b; Rashi). A stiff curtain is permitted, however, since there is no concern that anyone will wrap himself in it (Rambam; Tur; SA YD 301:11). Taz infers from this that one may not lean against the sha’atnez-covered side of a wagon, and Ḥokhmat Adam rules likewise (Taz YD 301:2; Ḥokhmat Adam 106:21). Panim Me’irot, however, permits leaning against sha’atnez fabric that is fixed in place so that it cannot be wrapped around the body. This is also the ruling of PT, Birkei Yosef, Kaf ha-Ḥayim, and Malbushei Yesha (Panim Me’irot 2:113; PT YD 301:1; Birkei Yosef YD 301:6–7; Kaf ha-Ḥayim OḤ 638:52; Malbushei Yesha 5:16).
Even stiff sha’atnez fabric may not be placed upon the body for protection, since protection is one of the functions of clothing. Thus, one may not place a donkey’s sha’atnez saddlecloth on one’s back in order to carry manure on it (SA YD 301:8).
