Today is Friday the 13th—a day thought to be unlucky because of the idea that number 12 is "complete" (think apostles, months of the year, zodiac signs) and 13 is just ... odd. That's the explanation given by Joe Nickell, a senior research fellow and paranormal investigator for Skeptical Inquirer magazine.

But in other countries, different numbers are thought to be just as unlucky. In case 13 makes you nervous, here are five other numbers to avoid.

4: In China, the pronunciation of the word for the number four is similar to that of the Chinese word for death. Many buildings in China skip a fourth floor, just as U.S. builders sometimes omit floor 13.

9: Just as the number four has a bad-luck soundalike in Chinese, 9 is feared in Japan because it sounds similar to the Japanese word for torture or suffering.

17: Some Italians are superstitious about Friday the 17th because rearranging the Roman numeral XVII can create the word "VIXI"—translated from Latin to mean "my life is over."

39: The number 39 gets a bad rap in Afghanistan. An NPR report explains: "Many Afghans say that the number 39 translates into morda-gow, which literally means 'dead cow' but is also a well-known slang term for a procurer of prostitutes—a pimp." So when Afghans see a car with number 39 on the license plate, they head the other way.

666: Hexakosioihexekontahexaphobia means fear of the number 666. In the Bible's apocalyptic Book of Revelation, John the Apostle refers to 666 as "the number of the beast." This "beast" is often interpreted as being the Antichrist—and thus the number is a sign of the devil.