Tzitzith need to be knotted to the four cornered garment. The commandment of tzitzith in Deuteronomy quoted in yesterday's Torah Thought came right after the commandment of Shaatnez. Yet tzitzith contains shaatnez! It specifically states that we are to put a wool thread (pesil) and this applies even if the garment is linen. Shaatnez must be wool and linen permanently attached together. The simplest stitch to permanently attach two threads is a double knot. So the attachment to the garment of the tzitzith is with a double knot, in order to effect the seeming violation of shaatnez that is overriden by tzitzith. This is considered to be oral tradition with just a hint in the Torah..
to be continued
- adapted from Tzitzith by Rabbi Aryeh Kaplan