The Biblical prohibition against mixing wool and linen.
In the middle of his rabbinical school admissions interview, a rabbi I know railed against those who "pick and choose" their Jewish practice. "Oh yeah," replied a member of the admissions committee, "that's a nice suit--have you checked it for shatnez?"
Implicit in the interviewer's comment was the recognition that even Jews who keep kosher and observe Shabbat often pay little attention to the prohibition against mixing wool and linen in the same garment. The strangeness of this law, combined with the difficulty of observing it fully, has led many Jews to subject this mitzvah to ridicule.
A Mysterious Word
Even the word "shatnez" is mysterious. The word doesn't look like a Hebrew one, and linguists have speculated that the term may come from the Coptic words "saht" (woven) and "nudj"(false) or from the Arabic words "shash" (black gauze) and "atmuz" (strong). Yet none of these explanations is universally accepted, and the origin of the word remains unknown.
The rabbis of the Talmud understood shatnezto be a condensed version of three Hebrew words--"shua" (combed or carded), "tavu'i" (spun), and "nuz" (woven), but this rabbinic answer serves more as a mnemonic device than as an explanation for the origin of the word (Mishnah, Kilayim 9:8). Other rabbinic suggestions for the origins of the word include classifying someone who disregards the laws of shatnezas "naluz (perverse) and turning God against oneself" (Sifra Kedoshim 2:4) or referring to wool "she'ma'ateh" (that covers) the sheep and "nutz" (sprout) of flax (P'sikta Zutarta Kedoshim 54a). All of these puns only highlight the difficulty of the word.
The Laws of Shatnez
Two biblical verses prohibit wearing shatnez. In both cases, the references to shatnez appear within the context of other prohibitions against kilayim--mixing two species together:
"You shall not let your cattle mate with a different kind; you shall not sow your field with two kinds of seed; you shall not put on cloth from a mixture of shatnez (Leviticus 19:19)."
"You shall not sow your vineyard with a second kind of seed. . . You shall not plow with an ox and an ass together. You shall not wear shatnez--wool and linen together (Deuteronomy 22:9-11)."
According to the talmudic rabbis, the Torah defined shatnez only as mixing linen made from the flax plant with wool from sheep or lambs.
The rabbis worried, however, that some less-common fabrics, such as silk, might be confused for flax, and therefore prohibited mixtures involving such materials out of fear of marit ayin--giving the appearance of breaking a law. When silk became more common, such concerns disappeared, and later legal authorities eliminated these prohibitions (Shulkhan Arukh Yoreh Deah 298:1).