In ancient times, two types of threads were attached to the corners of the tallit (prayer cloaks): from white wool (or from the material from which the man’s clothing was made) and from blue. The use of blue wool, called thelet, was a distinctive feature of the nobility and was consistent with the goal of the tallit to remind every Jew that he was a member of the “kingdom of the priests” of the Most High.

In the Torah, we read: "Speak to the sons of Israel and tell them to make themselves brushes (tzitzit) on the edges of their clothing in all their generations and weave a thread of blue wool into each brush of the edge." This verse contains two requirements. The first is to attach (white) brushes to the corners of a garment that has four corners, the second is to add a thelet thread to each brush. Moreover, these two requirements are independent of each other.

When we have a thelet, we need to add a blue thread to the tzitzit, and when not, we follow the command with the usual all-white brushes.

Unique thelet dye created with ink produced by a mollusk called chilazone, living in the Mediterranean Sea. So why today it is so rarely seen in the tallit or tzitzit thread thelet?

At a certain point in history, about 1000 years ago, the chilazone, which was always quite difficult to find, since it surfaced only once every 70 years, completely disappeared. And after some time, the skill of its extraction and recognition was completely lost, and since it became impossible to establish 100 percent exactly how this mollusk looked, the production of thelet stopped.

Throughout our history, there have been many people who have tried to rediscover the chilazone. The most famous among them were the Radzin Rebbe, Gershon Hanoch Leiner (1839–1891), and the chief rabbi of Israel, Yitzhak Herzog (1888–1959).

Leiner claimed that the mysterious chilazone is cuttlefish, and began to produce and distribute dye obtained from the ink of this marine creature. And Herzog, in the course of long research, was convinced that the mollusk Murex trunculus described by zoologist Henri Lacase-Dutier in the middle of the 19th century is much more suitable for description, and some still use its dye.

This conjecture was later confirmed by archaeologists excavating in ancient Tire. However, many still consider both conclusions to be very vague. In addition, as the Kabbalists write, the absence of thelet threads in our time corresponds to our weakened spiritual state. Therefore, most Jews continue to wear completely white brushes in anticipation of the arrival of Mashiach, when the prophet Eliyahu himself will lead us in search of a chilazone.