Mishnah Berurah — משנה ברורה

And there are those that say etc. - They rule like Rava [in the Talmud - Menachot 39b]: For Rava raises a contradiction: It's written regarding ציצית: "[And they shall put on the fringe of] the corner [a sky-blue thread]" (Numbers 15:38), which implies that [the material used for the ציצית strings must be from] the same type [of material] as the corner. Meaning that even if it's not from wool or linen, in any case the ציצית strings should be from the same material as the garment. And it's written [in another place]: "You shall not wear cloth combining wool and linen" (Deuteronomy 22:11)... [and the verse immediately after that says] "you shall make for yourself twisted cords [upon the four corners of your covering]” (Deuteronomy 22:12) [which implies that the ציצית strings must be] from them. (ie. either wool or linen.) How is this so? (ie. how do we resolve this contradiction?) [ציצית strings made from] wool or linen exempt [any garment], whether from their same material or from a different material. [ציצית strings made from] other materials exempt [only garments made from] the same material, but they do not exempt garments made from other materials. And the Rem"a leans towards/rules like this opinion since most of the later rabbinic authorities rule like Rava [to expound the words of the verses to be referring to] materials used for the corners... and learn from it that all materials used for garments are required on a Torah level to have ציצית strings and [that they should be] from the same material as the garment.