Whenever wool and linen are joined directly and securely, the garment is prohibited by Torah law as sha’atnez. Thus, a skirt in which wool and linen fabrics are sewn together, or a wool coat with a linen hanging loop or linen loops, is prohibited by Torah law.
If the wool and linen are not joined to one another, however, most authorities permit the garment. Thus, if wool fabric is sewn to one side of a cotton garment and linen fabric to the other, there is no prohibition, since the wool and linen are not connected. The same applies when pieces of wool and linen are placed inside a lining without being joined to one another (R. Shimshon of Sens; Ramban; Rosh; Rashba). Rambam, by contrast, maintains that once both materials are attached to a single garment, the entire garment is prohibited by Torah law. In practice, the accepted custom follows the lenient view, although the scrupulous are stringent (Rema YD 300:5).[5]
Sha’atnez is prohibited only when wool and linen are joined. When they remain separate, they may be worn together. One may therefore wear a wool shirt over a linen shirt and fasten a belt around both, wear a wool belt with linen pants, or fasten a wool garment to a linen garment with buttons that open easily or with Velcro. In each case, the garments are held together only temporarily and are not permanently joined (SA and Rema YD 300:3–4).[6]
Some maintain that one should not wear wool socks over linen socks, since the linen socks cannot be removed without first removing the wool socks and the two are therefore considered firmly joined (Rema YD 300:4). Most authorities permit this, however, because there is no permanent attachment. This is the accepted practice (SA YD 300:4; Levush; Shakh YD 300:8; AHS YD 300:24).
[5]. The Rishonim disagree whether wool and linen may be joined through an intervening material. The case they discuss concerns a wool garment reinforced and thickened with a leather covering. The leather was sewn to the garment with hemp thread, while the hem along its edge was sewn with linen thread. Rambam rules that “once the wool and linen have been joined in any manner whatsoever, it is kilayim by Torah law” (MT, Laws of Kilayim 10:2). Ra’avad and Orḥot Ḥayim agree, and several Aḥaronim take their stringent view into account (Mishkenot Ya’akov §70; Devar Shmuel §169; Minḥat Yitzḥak 8:117). Others maintain that joining them through an intervening material is prohibited only rabbinically because of marit ayin (Rav Hai Gaon; Ha-Eshkol).
Most Rishonim, however, permit this arrangement as long as the wool and linen are not attached directly to one another, even if the linen thread along the edge of the leather touches the wool. This is the view of R. Shimshon of Sens, Rivmatz, Ramban, Semag, Rashba, Rosh, and others. Rema records that the accepted custom follows the lenient view (Rema YD 300:5). This is also the ruling of Magen Avraham OḤ 9:8, Eliya Rabba OḤ 9:6, MB 9:19, and AHS YD 300:14.
[6]. Sewing wool fabric to linen fabric with hemp thread is prohibited by Torah law whenever the stitching creates a secure attachment. Thus, passing a needle through both fabrics three times is prohibited even without a knot, as is passing it through twice and tying a knot (Rosh; Rema YD 300:2). The same applies when the needle is passed once through the edges of the fabrics and the two ends of the thread are tied together with a double knot (Magen Avraham OḤ 11:13; Ḥavot Ya’ir §143, cited in PT YD 300:2). Some authorities are stringent even when the needle is passed through twice without a knot (MT, Laws of Kilayim 10:24; SA YD 300:2). The Aḥaronim disagree whether fastening wool and linen fabrics together with a safety pin constitutes prohibited sewing or is comparable to fastening them with a button and is therefore permitted (see Malbushei Yesha 3, note 38; Torat ha-Sha’atnez 300:32).
