Komentarz do Powtórzonego Prawa 22:11

שעטנז is an expression for a mixture. Our Rabbis explained it to mean a material that is calendered (pressed, שוע), or woven (טווי), or twisted (נוז) together (Sifrei Devarim 232:1); (שעטנז is taken to be an abbreviation of these words; cf. Rashi on Leviticus 19:19 and our Note thereon).

לא תלבש שעטנז, “you must not dress in a mixture of wool and linen threads.” Moses explains in this verse that this prohibition applies only to mixing wool and linen fibers. He mentions the word לבש to wear, to indicate that only when such mixtures are worn as a single garment is this probation applicable. Wearing a linen shirt and a woolen suit is perfectly in order. Also, even if such a mixture is used, say protectively, but not in the manner one wears clothing, one has not transgressed this commandment.

.לא תלבש שעטנז צמר ופשתים יחדו , “Do not wear a mixture, i.e. wool and linen together.” The Torah forbids this mixture only when woven together or sewn together. This is the meaning of יחדו. It is perfectly in order, however, to wear a linen shirt underneath a woolen jacket (Maimonides Hilchot Kilayim 10,11). It is also permitted to sew together the skins of animals producing wool with linen threads, seeing that this does not constitute a garment. We discussed the mystical aspects of this commandment in Leviticus 19,19.