On March 17, people (and buildings and beer) around the world get decked out in green to flaunt their Irish pride and celebrate St. Patrick’s legacy. But take a look at the earliest depictions of the famous Catholic missionary and you’ll notice something strange: He’s typically shown wearing blue, not green.

As difficult as it is to imagine, Ireland wasn’t always associated with the classic secondary color. Smithsonian.com reports that King Henry VIII sought to cement England’s centuries-old reign over the island nation by declaring himself the King of Ireland in 1541, and he even gave it a new coat of arms: a gold harp against a blue background. More than 200 years later, when King George III established an order of knights called the Order of St. Patrick, the color blue’s connection to Ireland became even more pronounced: the order’s official color was a shade of sky blue that came to be known as “St. Patrick’s blue.”

Meanwhile, Irish nationalists were looking for ways to separate themselves—politically and chromatically—from the English. According to TIME, the color green first appeared on the scene during the Great Irish Rebellion of 1641, when military commander Owen Roe O’Neill brandished a green flag with a harp to represent the Confederation of Kilkenny, which was trying to put an end to Protestant control of the region. Then, in the 1790s, the Society of United Irishmen—a revolutionary group advocating for republicanism—donned uniforms that consisted of green shirts, green and white striped pants, and felt hats with green cockades (or rosettes).

The era also gave rise to patriotic poems and ballads, many of which used the color green—and Ireland’s rich natural landscapes—as an emblem of Irish pride and resilience. In the popular ballad “The Wearing of the Green,” for example, the lyrics state that “You may pull the shamrock from your hat and cast it on the sod / But 'twill take root and flourish there, though underfoot 'tis trod.” In 1795, co-founder of the Society of United Irishmen William Drennen first referred to Ireland as “the Emerald Isle” in his poem “When Erin First Rose,” writing “Let no feeling of vengeance presume to defile / The cause of, or men of, the Emerald Isle.”

Over time, green became strongly symbolic of Ireland in general, and St. Patrick’s use of the (green) shamrock to explain the Holy Trinity in his teachings had a more lasting influence than his association with blue from the Order of St. Patrick. Green landed on Ireland’s national flag in 1848 and, as droves of Irish immigrants arrived in America throughout the 19th century, they brought with them the tradition of wearing green to celebrate his feast day.

Those celebrations didn’t always happen at pubs or involve alcohol, though—find out why here.

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