The Torah commands in Parashat Ki-Teseh (Debarim 22:11), “Lo Tilbash Shaatnez, Semer U’fishtim Yahdav” – “You shall not wear Shaatnez, wool and flax together.” This introduces the Biblical prohibition against wearing a garment that contains a blend of wool and flax, a combination known as “Shaatnez.”
The Rambam writes that the Torah forbade wearing Shaatnez because it was customary among idol-worshippers – and especially the pagan priests – to wear garments prepared from wool and flax. In fact, the Rambam adds, the priests during his time in Egypt wore Shaatnez garments. As part of the Torah’s effort to distance Beneh Yisrael from pagan beliefs and practices, it forbade wearing Shaatnez, a type of garment associated with idolatry.
Others explain the Shaatnez prohibition based on the story told in Parashat Bereshit of Kayin and Hebel, the two sons of Adam, who both offered sacrifices. Kayin’s sacrifice was rejected, whereas Hebel’s offering was accepted, causing Kayin to feel such jealousy that he killed his brother. The Torah relates that Kayin brought a “Korban” (sacrifice), and the letters that spell the word “Korban” are Kof, Resh, Bet and Nun. The last letters of the names of these four letters are Peh, Shin, Tav and Nun, which spell the word “Pishtan” (flax) – alluding to the fact that Kayin, who worked as a farmer, brought an offering of flax. Hebel, the Torah tells, worked as a shepherd, and he brought sheep as his offering. A combination of wool and flax thus bring to mind the first fight in human history, which led to the first homicide in human history. In order to remind us of the dangers of jealousy, competition and strife, the Torah warns against combining wool and flax.
Ultimately, however, the prohibition of Shaatnez is a “Hok,” a Misva whose rationale eludes our understanding, and which we nevertheless observe in humble subservience to the Almighty.
The Sages understood the word “Shaatnez” as alluding to the three conditions that must be met for a garment to be considered “Shaatnez” and thus forbidden to be worn by Torah law: “Shua,” “Tavi,” and “Nuz.” “Shua” means that the material – both the wool and flax – was combed; “Tavi” refers to the material’s having been spun into threads; and “Nuz” means that the threads were woven together. If the wool and flax underwent these three processes, and the two were then woven together, or tied together by two knots, then the garment is forbidden on the level of Torah law. The Sages enacted a prohibition forbidding a garment if even only one of these three stages were completed.
If ten mats are piled on top of one another, and the bottom one is made from Shaatnez, one may not sit on the pile, given the possibility that a thread of Shaatnez will wrap around his foot, and he will then be in violation of the Shaatnez prohibition.
One violates this prohibition only if he wears Shaatnez as a garment, in a manner which can be considered “wearing,” the way garments are worn. The Sefer Ha’hinuch writes that if a Shaatnez garment is stiff, such that it is not comfortable to be worn, it is not forbidden. The Sefer Ha’hinuch speaks of a certain type of felt hat that people would wear to shield themselves from the sun, like a vizor, and Rabbis permitted wearing this hat even though it was Shaatnez, because the material was firm and stiff.
Even a single thread of Shaatnez in a garment renders it forbidden to be worn.
This prohibition applies in all times and places, and is binding upon both men and women. One who wears a garment containing Shaatnez is liable to Malkut. Even if he wears the garment an entire day, he receives only one set of Malkut. However, if he was warned on multiple occasions, and ignored the warnings, then he is liable to a separate set of Malkut for every warning that he disregarded.
The Zohar comments that the prayers recited by somebody wearing a Shaatnez garment cannot be answered. It is therefore especially important when buying new clothing for Rosh Hashanah and Yom Kippur, when so much depends on our prayers, to ensure that the garments do not contain Shaatnez.