The Torah said, "You shall not wear shaatnez, wool and linen together. [But] You shall make yourself braided fringes on the four corners of your garments." This teaches that although shaatnez is forbidden, in tzitzit it is allowed. Moreover, one cannot say that the linen and wool are simply put together, because the tzitzit must have at least one double knot, which makes it permanently attached, and thus part to the garment.
The braiding should be done with no less than seven loops, to remind one of the seven heavens, and no more than thirteen loops, to correspond to the seven heavens and the six spaces between them.
All four-cornered garments require tzitzit. Rav Nachman exempts the silk garments. But we learned that they too require tzitzit?! - Says Rav Nachman, "By the decree of the Sages, to make it look good."
Art: Caspar David Friedrich - Moonrise over the Sea