If the number 13 scares you, you’re not alone.

Millions of people in the world, including prolific horror writer Stephen King, have an irrational fear of the number 13. The phenomenon is so widely reported, it even has its own hard-to-pronounce name: triskaidekaphobia.

Those who suffer from triskaidekaphobia associate the number 13 with bad luck or danger due to superstitions. They may avoid staying at hotel rooms with the number 13, going up to the 13th floor of any building or sitting in the 13th row in airplanes — if such floors or aisles even exist.

People with more deeply rooted triskaidekaphobia, like King, might also skip the 13th step on staircases, get anxious watching Channel 13 or, while reading books, make a point not to pause on pages in which the digits add up to 13, like page 94. “It’s neurotic, sure. But it’s also . . . safer,” King wrote about his phobia in 1984.

It’s hard to quantify how many people in the world fear the number 13, since the phobia often goes undiagnosed and untreated, according to Reid Wilson, a clinical psychologist and adjunct associate professor of psychiatry at the University of North Carolina School of Medicine who specializes in treating anxiety disorders like phobias. But the phenomenon has had clear economic implications in at least the real estate, airline and entertainment world. “We have such a cultural sense of trepidation about that number,” Wilson told TIME.

You won’t find a row 13 on any Ryanair plane, the Dublin-based carrier confirmed in a tweet to a customer in 2014. And many other airlines and airports avoid slapping the number 13 on aisles, flights and gates — sometimes out of logistics and other times because of triskaidekaphobia, according to the Wall Street Journal.

The two largest elevator makers in the world, Otis and Kone, say they both offer building owners an option to omit a number 13 button in elevators. And at least in New York City, less than 5% of residential condo buildings in Manhattan and Brooklyn had a 13th floor in 2013, the Journal reported at the time. “I’m not particularly superstitious myself, but not having a 13th floor is a no-brainer,” building developer Izak Senbahar told the newspaper. “You don’t want to preclude anyone, a buyer who happens to be superstitious. It boils down to that.”

For “triskies,” as King calls them, the number 13 is scary. But when the date falls on a Friday, it’s horrifying. Such a double-whammy of fears has its own name: paraskevidekatriaphobia. Millions of Americans may suffer from fear of Friday the 13th, according to Saybrook University psychology professor Stanley Krippner. Hollywood capitalized on that by spawning the Friday the 13th horror movies franchise, in which the ominous day is associated with a nightmare-inducing serial killer.

Naturally, fears could worsen when a Friday the 13th falls in October around Halloween, which it does this week. “We as a culture have established Friday the 13th as something that could be dangerous,” Wilson said. “All day long they can be on edge.”

It’s a mystery how such fears became rooted in modern culture, folklore historians say. Dr. Phil Stevens, an associate professor of anthropology at the University at Buffalo, told TIME last year that many believe the superstition originates from the Last Supper, when 13 guests sat at the table with Jesus on the day before the Friday on which Jesus was crucified. Other historians and evolutionary psychologists chalk it up to people needing a scapegoat when life doesn’t go their way.

“People are hard-wired to find meaning in various patterns, connections and perceptions,” Krippner said. “They need someone or something to blame when stuff goes wrong, and numbers are an easy target.”

The good news is this week’s Friday the 13th is the last one this year. The next one won’t happen until April 2018 and then later that July.

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