Question
I heard someone talk about a "kosher suit". How can there be such a thing?

Answer
"Kosher" clothing contains no sha'atnez, the mixture of wool and linen. This is one of several Biblical laws about forbidden mixtures (Leviticus 19:19, Deuteronomy 22:9-11).

According to the Mishnah (Kilayim 9:8), sha'atnez denotes three processes, shu'a (combing), tavi (spinning) and
There have been many attempts to discover the rationale behind the sha'atnez law.
nuz (twisting). The Biblical law applies if all three steps have been employed. Other combinations - e.g., wool and linen, which have only been pressed into felt - are prohibited only by rabbinical decree, though Maimonides holds that they too are covered by the Biblical law.

Even one thread anywhere in the garment can create sha'atnez, which applies not only to normal garments but to items which easily wrap around or rest on part of the body; e.g., towels, pillows and blankets. However, there is no problem if a person is wearing two separate pieces of clothing such as a woollen suit and a linen shirt, as they are not combined into one garment.

Chemical and microscope tests can easily identify wool and linen fibres; and if there is a problem of sha'atnez it can usually be remedied with some minor tailoring work. Sha'atnez does not apply to synthetic fibres, though sometimes even 100% polyester suits can contain linen in the padding, collar supports, seam bindings, etc.

There have been many attempts to discover the rationale behind the sha'atnez law. One view is that it was God's intention at the time of creation that every species or category should preserve its own distinct identity. The Midrash associates the law with the story of Cain and Abel. Linen, coming from the flax plant, recalls Cain's offering; wool, from the sheep, is a reminder of Abel's, and the meeting of the two represents the encounter between the two brothers which led to jealousy and murder.

In ancient days a kohen was allowed to wear sha'atnez because only he, as God's representative, could reconcile and synthesise all diverse elements.