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According to the Shaatnez testers of America
Suits and Sport jackets, (Men's/Boy's): Require testing even 100% polyester and 100% silk suits.
Why so? Isn't there a simple Sefek Sefeka? Safek if there is wool, and if there is wool Safek if there is Linen! This is even a Safek Sefeka HaMithapech. Why would they require testing?
(I'm not looking for an answer arguing against the entire practice of checking for Shaatnez)
The 100% in 100% polyester and 100% silk suits refers to the visible part of the suit.
Underneath that is an entire world of stuffing, reinforcement and other junk. The easiest place to see some of these is under the lapel; under the felt is something stiff, and it's not silk/polyester.
These hidden parts are made from whatever material the factory can buy at the cheapest price. It usually differs even from one identical suit to the next; depending on what came out of the sack next.
Shaatnez checkers will tell you that it's not at all unusual to find both wool and linen in these pieces, maybe even material made from both.
There's also the issue of the thread used for sewing on the buttons. Linen thread is still one of the strongest threads available.
BTW: Have you ever seen a zipper of 100% polyester or 100% silk?
To answer your question: There is only one Safek: Do the non-silk/polyester sections contain wool and linen?
Disclosure: My father was a Shaatnez checker for over 30 years.
