Not to wear Shatnez, a cloth woven of wool and linen

This mitzvah prohibits wearing clothing woven from both wool and linen, known as Shatnez. While the Torah allows both materials individually, their combination in one garment is forbidden. This prohibition is part of the larger framework of kilayim (forbidden mixtures) that also applies to agriculture and animals.
The mitzvah is a chok (statute) — its reason is not fully explained, though commentators suggest it instills reverence for Hashem’s decrees and preserves symbolic distinctions in creation. Exceptions exist only in specific mitzvot (e.g., priestly garments, tzitzit according to some opinions), which demonstrate that sanctity is defined by Divine command, not by human logic.

Commentary & Classical Explanation:

• Rambam (Hilchot Kilayim 10:1): Defines Shatnez as a mixture of wool and linen spun, woven, or joined together. Wearing even a thread’s worth in combination is prohibited.
• Sefer HaChinuch (Mitzvah 551): Explains that the root is to train us in obedience to Hashem’s laws, even when no reason is apparent, and to preserve separations in creation.
• Talmud (Niddah 61b, Yevamot 4b): Discusses details and rare exceptions, such as garments of kohanim.
• Rashi (Deuteronomy 22:11): Notes that the Torah singles out wool and linen as incompatible for ordinary wear, highlighting the Torah’s authority over material distinctions.