Triskaidekaphobia is an irrational fear of the number 13. It is usually considered a superstition.

It has been linked to the fact there were 13 people at the last supper of Jesus, but it probably originated only in medieval times. It has also been linked to that fact that a lunisolar calendar must have 13 months in some years, while the solar Gregorian calendar and lunar Islamic calendar always have 12 months in a year.

Triskaidekaphobia may have also affected the Vikings - it is believed that Loki in the Norse pantheon was the 13th god. This was later Christianized into saying that Satan was the 13th angel.

The Mesopotamian Code of Hammurabi (ca. 1686 BC) omits 13 in its numbered list. This seems to indicate a superstition existed long before the Christian era.

Some buildings number their floors so as to skip the thirteenth floor entirely, jumping from floor 12 to floor 14 in order to avoid distressing triskaidekaphobics, or using 12a and 12b instead. This is sometimes applied to room numbers as well.

A specific fear of Friday the 13th is called paraskavedekatriaphobia.

One famous figure to have suffered from triskaidekaphobia was the composer Arnold Schoenberg. He was born and died on the 13th of the month.

American singer-songwriter John Mayer had 14 tracks on his album Room for Squares, although the 13th is 0.2 seconds of silence and is not listed on the album cover.

In spite of all this, thirteen can prove to be quite lucky for many people. Trivia seekers can give testimony to the presence of 'Thirteens' in many things, including the US 1 Dollar note, with

13 levels of the truncated pyramid
13 stripes on the flag
13 letters in "E Pluribus Unum"
13 stars above the Eagle
13 leaves on the olive branch
13 arrows held by the Eagle
13 bars on the shield.

Actually, these are supposed to represent the 13 original Colonies in the United States of America.