Kilayim, Chapter 9, Mishnah 7. Our Mishnah lists various items that a person may not wear until he has checked whether they contain a mixture of sha'atnez. The items themselves belong to those times and are not practically relevant to us, but they serve to teach us the very obligation to check garments for sha'atnez.
The words of the Mishnah:
"HaBirsin v'haBardesin" - two types of bed coverings that were made of wool: one thick and the other thin.
"V'haDalmatikiyon" - a type of trousers that people commonly wore.
"U'min'al shel Alus haPinon" - a shoe that was made from a type of wool by-product.
Regarding these items the Mishnah rules: "Lo yilbash bahen ad sheyivdok" - one may not wear them until a check is made as to whether they contain linen, that is, whether the garment contains a mixture of linen together with the wool.
The opinion of Rabbi Yosi:
Rabbi Yosi says that garments that come from the seacoast or from overseas do not require checking, "mipnei shechezkatan b'kanbos" - one may assume that in those places linen was not used, but rather hemp in its place, and hemp poses no problem of sha'atnez. This ruling too belongs specifically to their times.
"Min shel Zerad":
Some have the reading "min shel zerav": a leather shoe that was lined on the inside with a thick padding of wool, and these shoe coverings rose up and covered the foot and also part of the leg. Concerning this the Mishnah rules that it does not involve kilayim, meaning that there is no concern of kilayim in it - and again, this ruling is unique to those times and to that particular garment.
In summary: In this Mishnah we learned a list of items - the bed coverings (habirsin v'habardesin), the dalmatikiyon and the shoe of Alus haPinon - which may not be worn until they are checked for a mixture of linen; the opinion of Rabbi Yosi that garments from the seacoast and from overseas are exempt from checking because they are presumed to be of hemp; and the ruling that a "min shel zerad" involves no kilayim. The details indeed belong to those times, but the principle that emerges from them stands firm: the need to check one's garments for sha'atnez.