Red, white, and blue barber poles look patriotic, but there's a blood-soaked meaning behind their design.

Those rotating red, white, and blue poles outside barbershops have become an icon. At first glance, you’d probably assume barber pole designs have a patriotic background. But the reality is pretty gruesome.

Barbers have been cutting hair for centuries, but they used to have a longer job description. In medieval times, the professionals were known as barber-surgeons, which is just what it sounds like. They weren’t just there to give customers a trim—they’d also perform minor surgery, pull teeth, and amputate limbs, according to PBS. But one procedure—bloodletting—led to the barbershop poles you see today. Want to learn more about the past? These are the 16 history questions everyone gets wrong.

At the time, people thought having too much blood in a certain area could cause diseases like fevers or the plague, and letting some out would make them healthy. In 1163, Pope Alexander III ordered monks and priests to stop performing bloodletting anymore, so barbers started offering the service instead, according to History.

During the treatment, barber-surgeons would give patients poles to hold, the original barber poles. Grasping the staff made their veins pop out a bit, making them easier to find while the barbers went all Sweeney Todd.

Even back then, people knew there was a limit to bloodletting, so barbers would stop the bleeding with a white cloth. They’d then tie those towels to the poles and hang them outside their shops, according to History. Some towels stayed blood-stained even after they were washed, so it was common to see a pole with white and red swirling around in the breeze.

These days, barbers leave the medical treatment to doctors, but their poles are a nod to their bloody past. In Europe, barber poles are just red and white—reminiscent of the poles from the Middle Ages. There are a couple of theories about why the United States added blue to its design. Some say it represents the blue of the veins barber-surgeons would target when bloodletting; others think it’s just copying the American flag as a sign of patriotism. Learn more of the biggest lies in history.

Some places are very selective about who’s allowed to display a barber pole. In certain states, it’s against the law to have one outside a salon unless a licensed barber works there. Licensed or not, though, we’re just happy to know there won’t be any bloodshed.