Kilayim 9:9-10

Kilayim 9:9

Felt (which is pressed wool) may not be combined with linen because they would be combed together. A border of wool on a linen garment is prohibited because the threads overlap the cloth. Rabbi Yosi says that cords of purple wool may not be used with a linen garment because one sews them together to keep them from being separated. One may not tie a wool ribbon to a linen ribbon to use as a belt, even if they are separated by a leather strap in between them (because one will tie the two ends together when he girds himself with it).

Kilayim 9:10

Identifying marks sewn onto a garment present a problem of shaatnez (such as embroidering one’s initials on a wool garment using linen thread or vice versa). One stitch is not considered a connective (to render one garment impure if another is attached in such a fashion), nor does it present a problem of shaatnez (if a wool and linen garment are connected with a single stitch). If one undoes such a stitch on Shabbos, he is exempt from punishment (though that is not to say that it is permitted to do so). If one brings both ends of the thread to the same side (i.e., two stitches), it is a connective, it does render shaatnez, and one would be liable for undoing it on Shabbos. Rabbi Yehuda says it takes three stitches to create these effects. A sack with a wool lining and a basket with a linen lining form shaatnez with two stiches (and may therefore not be carried on the shoulder).
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