Most of the western world such as the United States, Canada, Australia, the United Kingdom and Europe has a history of considering 13 to be an unlucky number. The fear of 13 as an unlucky number is called triskaidekaphobia. The association with thirteen and bad luck is thought to be tied to Christianity and The Last Supper.

Jesus Christ and his 12 followers sat at the dining table at The Last Supper, making a total of 13 people. One of the followers betrayed Jesus and the next day, a Friday, Jesus was crucified. Friday the 13th is considered to be an especially unlucky day. Some travelers still refuse to travel on that day. Some hotels were built without a thirteenth floor due to 13 being considered such an unlucky number.

Norse mythology also includes the idea that thirteen is an unlucky number of people at a dining table. Loki, god of evil, was the thirteenth guest at a table that was supposed to seat only 12 gods. Loki joined the table uninvited and was responsible for the accidental death of the god Baldur.

Although the ancient Romans thought that 13 was an unlucky number, they believed that 2 was even more of an unlucky number than 13. The ancient Romans associated thirteen with death, but two with Pluto, the god of the underworld. Pluto was thought to hold the second day of the second month of the year as a sacred day.

Most Asian countries do not consider 13 to be an unlucky number, but rather China, Japan, and Korea traditionally see the number 4 as being a very unlucky number. The sounds spoken to mean the number four are very similar to the sounds meaning "death." Marketing studies conducted in China found that product names and labels with the number 4 did not sell nearly as well as similar items with the number 8 on them instead. Eight is traditionally seen as the luckiest number in China.