1. The prohibition of Shatnez involves not wearing wool and linen that were woven together in one fabric. The prohibition is mentioned in the Torah and is one of several prohibitions against intermixing separate kinds of materials.
2. The source of the word Shatnez according to our sages is an acronym of Shua, Tavei and Noz (mixing and weaving together threads). In Kabbalah, they explain the name as coming from the words Satan Az (brazen Satan).
3. The prohibition of Shatnez is mentioned in the Torah in two places. In Leviticus (19:9) it is mentioned in the context of other prohibited mixtures. Deuteronomy (22:11) explains that a Shatnez garment is made of “wool and linen together.”
4. According to the sages, there is no prohibition to wear wool and linen threads together unless they are interwoven.
5. One has to be careful that clothes and coats, especially those made of wool, do not violate the prohibition of Shatnez. One should also check suits and shirts, trousers and hats, scarves and coats, clothes and furniture, tablecloths, bedspreads, mattresses and even sheets, sofas, blankets and more. According to most government laws, there is no requirement to specify a fabric ingredient if it is below 5%, but the Torah does not even allow a forbidden mixture of even one thread. So you can not rely on the labels that come with garments.
6. Today there are special laboratories run by pious textile experts that test whether the garment has a mixture of wool and linen.
7. Shatnez laboratories which test for shatnez often use a microscope to identify all the components of the fabric to confirm that the garment does not contain both materials.
8. The sages say that we do not know the reason for this prohibition and have to keep it because it is the King’s decree. Maimonides states that the idolators would wear shatnez clothes so the Torah forbade it.
9. What is the reward of one who is careful with shatnez? The book Chachmat Adam guarantees that “all who are careful not to wear shatnez will merit to wear clothes of salvation and a coat of righteousness.” The Pnei Zaken commentary promises that “the one who was careful from all forbidden mixtures will live long in this world and the Afterlife, will not return in another reincarnation, will rest in peace on his bed, beget righteous sons who will live and be well, and will merit to get everything good in This World. The Komarna Rebbe in his commentary on the Torah states that “he merits the light of life, the light that is sown for a righteous man, and his home will be full of G-d’s blessings and kindnesses and everything good.”
10. The prohibition is very severe and one who wears shatnez transgresses a Torah prohibition every moment. The Drushei ha-Tzelach says that if one wears shatnez even a single day, his prayers are not heard for another 40 days.