SHA'ATNEZ (Heb. שַׁעַטְנֵז; Gr. κίβδηλος, "counterfeit"), cloth combining wool and linen. Leviticus 19:19 and Deuteronomy 22:11 prohibit the wearing of sha'atnez, in the former passage explained as beged kilʾayim, "cloth made from a mixture of two kinds of material," in the latter passage explained as "wool and linen together." While the meaning of the term is therefore clear, its etymology is obscure, modern speculation ranging from unlikely Hebrew combinations (e.g., saʿarat-aʾnez supposedly meaning "[sheep's] hair-[flax] stalk") to the no more likely Coptic (saht, "woven," nudj, "false" [cf. the LXX]), and vulgar Arabic (shash, "black gauze," ʿatmuz, "strong") derivations. The word looks foreign.
The clothing of the priests was notably exempt from the prohibition of sha'atnez. Exodus 28:6, 8, 15, and 39:29 prescribe that various pieces be made of linen and colored wool interwoven (cf. Kil. 9:1: "Priests wear only wool and linen [i.e. sha'atnez] when they serve in the temple"). This suggests that the general prohibition was grounded on the taboo character of such a mixture, pertaining exclusively to the realm of the sacred (cf. Maimonides' view, below).
The rabbis interpret the word sha'atnez as being a compound standing for shu'a (שׁוּעַ; each thread smoothed out by the process of carding); tavui (טָווּי; each strand spun); and nuz (נוּז; woven or twisted together). The Torah's prohibition against sha'atnez therefore only applies when a strand of wool and one of linen, each carded, spun, and twisted, have been joined together by weaving, sewing, or tying (Kil. 9:8). The rabbis, however, prohibited the wearing of wool and linen even when their threads are simply sewn, tied, or pasted together (Nid. 61b; Rashi to Hor. 11a). The prohibition was extended to include sitting on sha'atnez fabrics although the Torah originally only forbade the wearing of mixed garments (Ta'an. 27b). It is, however, permitted to utilize sha'atnez shrouds for a corpse (Nid. 61b).
In accordance with the general principle that a positive precept overrides a negative one, it is permitted to attach blue woolen ẓiẓit to a linen garment (Men. 40a). It was likewise permitted for priests to wear garments of mixed texture prescribed by the Torah when performing priestly service in the Sanctuary (Yoma 69a).
Although the prohibition is considered a prime example of a divine statute which has no rational explanation (חוֹק), Jewish thinkers throughout the ages have attempted to rationalize its intent. Maimonides explained that the wearing of mixed garments was forbidden since heathen priests wore such garments (Guide of the Perplexed 3:37). Naḥmanides suggested that the person mixing diverse kinds was guilty of displaying that he was improving upon the species created
by God (Commentary to Lev. 19:19) while S.R. Hirsch saw in this commandment a reminder to man that he must guard his assigned purpose and place in the world just as the species must be distinctly preserved (Commentary to Lev. 19:19, tr. by I. Levy (1958), 534f.).
Elaborate chemical and microscopic tests have been developed to check for the presence of sha'atnez in clothing. In many cities with large Jewish populations, sha'atnez laboratories have been established to check for sha'atnez in garments.