are the thready fringes that hang from the corners of a tallit. The tzitzit we use today are almost uniformly white, but according to Jewish tradition, they’re allowed to be any color of the rainbow–except blue.
In the Torah, we are told to “bind a thread of blue to each corner” of tzitzit. This color, called techelet, is a very specific shade of blue. The color comes from a dye which is extracted from an animal known only–in Ezekiel 27–as the chilazon. However, during the Babylonian Exile, the knowledge of which animal that was, exactly, was lost.
In modern times, starting with the Radziner Hasidic rebbe in 1887, several people have used close examinations of ancient texts and chemical sampling of old tzitzit to determine which cephalopod, if any, is a chilazon. While there is no unanimously accepted contemporary techelet, some people have started dying strands of their tzitzit with what they believe techelet to be. Gradually, the swish of blue is becoming a more and more common occurrence in synagogues.