A web search for “Hanukkah decorations” yields a trove of whimsical items like menorah-and-dreidel-adorned straws, pillows, mugs, candy and napkins.

These widespread choices have one thing in common — they are blue and white.

How did blue and white come to represent Judaism in spaces from the Israeli flag to the Hanukkah gift aisle at Target?

READ: Hanukkah Celebrations Have Changed Dramatically

Like the Jewish religion itself, the answer is some combination of ancient tradition and modern adaptation.

An ancient dye

Scholars believe the story begins with mention, at a key moment in the Hebrew Bible, of an indigo blue dye called t’chelet. 

Religious historians and scientists believe t’chelet is derived from murex trunculus, a medium-sized sea snail that is as abundant today in the Mediterranean Sea as it was in ancient times. The snail secretes a chemical substance that turns indigo blue when exposed to the ultraviolet rays of sunlight.

The word t’chelet appears in the Book of Numbers, the fourth of five books that comprise the sacred Jewish text called the Torah. It describes “a cord of blue” that ancient Israelites were instructed to attach to a fringed garment they were commanded to wear as a sign of the covenant between God and the Jewish people.

To this day, Jewish people wear a fringed shawl, called a tallit, when praying or in daily life. Though a tallit can be adorned with designs or colors, typical shawls are white with blue or black stripes; some include a single blue fringe at each corner, as described in the Torah.

A costly history

In the ancient Near East, the dye was highly valued across multiple cultures.

It was “the most expensive commodity that existed,” said Rabbi Dr. Ari Greenspan, who helped found a non-profit organization, Ptil Tkhelet, to raise awareness of the ancient dye so Jews can fulfil the biblical commandment as precisely as possible.

Greenspan said Roman leaders restricted its use to ruling classes. The dye was costly well beyond its financial worth; writings from Roman rulers including the 5th century emperor Justinian record that anyone caught wearing blue or purple would be put to death.

Within Judaism, t’chelet risked being lost to history. The last reference to t’chelet in Jewish writings is around the year 750 C.E., according to Greenspan.

“At some point, the knowledge needed to do it was simply forgotten,” he said.

Centuries later, in the early 20th century, Jewish scientists — including Rabbi Isaac Herzog whose grandson and namesake is the current president of Israel — conducted research on various mollusks and squid species to identify the precise source of the biblical commandment so Jews can fulfil it.

Because of this work, said Lynette Nusbacher, a military historian with expertise in biblical warfare, “it’s one of the few [biblical] colors we can identify today with some reliability.”

Modern-day blue and white

Herzog’s research came at a time when the colors re-emerged in widespread association with Judaism through the nascent American Zionist movement to establish a Jewish homeland.

The Jewish National Fund’s iconic “Blue Box” first appeared in 1901 as a recognizable way for American Jews to raise money to purchase land in modern-day Israel.

Blue and white also distinguished the “flag of Judah,” which was first displayed at an American Zionist conference in Boston in 1891.

At one time there were 164 variations of the flag, according to Jonathan Sarna, a professor of American Jewish history at Brandeis University. All versions, including one that hung in the Hall of Nations at the 1904 World’s Fair in St. Louis, included blue marks against a white field.

The national flag of Israel, adopted in 1948, is white, with two parallel blue stripes along its long edges and a blue Star of David in the center.

According to Sarna, “the colors served myriad purposes,” including “the desire to give Jews a proud identity among the family of nations, all of whom had colored flags.”

Nusbacher said blue and white flags were flown in British-occupied Palestine. During the Second World War, the British Army’s Jewish Brigade Group’s uniform included a blue and white striped patch with a gold Star of David in the center—a strong contrast to the gold star the Nazis imposed on European Jews.

An ocean of symbolism

Unsurprisingly, given the colors’ long history, symbolic interpretations vary widely.

An 1864 verse by the Austrian Jewish poet Ludwig August Frankl says, “White is the radiance of the priesthood, and blue the splendors of the firmament.”

Some Hasidic Jews say the blue represents God’s might, while the white represents God’s mercy.

And some scholars, noting that the color changed its hue in daylight, believe the blue thread was used to help people tell time or know when to pray at the precise moment of daybreak.

“It’s an amazing topic,” Greenspan said, “it touches physics, art history, world history, and culture. It’s literally an ocean.”

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Holly Lebowitz Rossi is a freelance writer based in Arlington, Massachusetts. A graduate of Harvard Divinity School and a veteran religion reporter, she is also coauthor of The Yoga Effect: A Proven Program for Depression and Anxiety.