You might notice when you enter a hotel that the elevator doesn’t have a 13th floor button. This isn’t just a matter of superstition but rather a practical decision that allows buildings to be more efficient and averts potential problems.

The number 13 has always been considered unlucky. The superstition is believed to have grown over the years and perhaps picked up momentum following the release of the movie Friday the 13th in 1980. Since that time, the belief has become a widespread phenomenon and many hotels avoid having a 13th floor for this reason.

According to a 2007 Gallup poll, about 13% of Americans would be bothered by being given a room on the 13th floor and close to 10% of those who are bothered by it say they’d ask to be moved to another room. Triskaidekaphobia, a legit phobia of the number 13, can cause symptoms such as fear or anxiety and it’s estimated that about 10% of the population suffers from it.

Some apartment buildings and high-rise skyscrapers skip the 13th floor, but others don’t. For example, the iconic Empire State Building has a 13th floor and so does One World Trade Center. The Otis Elevator Company estimates that about 85% of the buildings it installs elevators in don’t have a 13th floor button. It’s also common to see 13th floors missing in hospitals and even at airports. But why is that?