The Torah prohibition of Sha'atnez forbids wearing a garment that contains both wool and linen. Does this prohibition also forbid wearing wool and linen garments one on top of the other? For example, if a person wears a linen shirt, may he wear a woolen Talit Katan underneath the shirt? Or, if a person in the wintertime wishes to wear two pairs of socks, may he wear a woolen sock underneath or on top of a linen sock?
This issue is subject to a debate between the Shulchan Aruch and the Rama (Rabbi Moshe Isserles, Poland, 1525-1572). The Shulchan Aruch (Yoreh Dei'a 300:4) rules that one may wear a linen garment together with a wool garment even if the two are tied together, provided that they are not sewn together with two or more stitches. The Taz (commentary to the Shulchan Aruch by Rabbi David Halevi, 1586-1667) understood that when the Shulchan Aruch permits wearing wool and linen garments tied together, he refers only to wearing a belt around the two garments. According to the Shach (commentary to the Shulchan Aruch by Rabbi Shabtai Ha'kohen, 1621-1663), however, the Shulchan Aruch permitted wearing the two garments even if they are actually tied together, provided that they are not sewn together.
The Rama disputes the Shulchan Aruch's ruling entirely, and forbids wearing a woolen garment and a linen garment if one cannot be removed until the other is removed. He thus forbids wearing a linen sock over a woolen sock (or vice versa), since one cannot be removed without the other. According to the Rama's view, it would likewise be forbidden to wear shoes containing linen while wearing woolen socks. It is recorded that the Vilna Gaon (Rabbi Eliyahu of Vilna, 1720-1797) followed the Rama's position and made a point never to wear woolen Tzitzit, since he would occasionally wear linen shirts.
As for the final Halacha, Hacham Yishak Yosef (as recorded in Yalkut Yosef Yoreh Dei'a, vol. 3) ruled in accordance with the Shulchan Aruch's position, allowing one to wear a woolen garment over or under a linen garment, provided that they are not sewn together with two or more stitches.
Summary: One may wear a woolen garment over or beneath a linen garment, even if they are tied together, provided that they are not sewn together with two or more stitches.
