The number 13 is often considered unlucky in many cultures, leading to the superstition known as triskaidekaphobia. This fear can manifest in various ways, such as the omission of the 13th floor in buildings or the avoidance of scheduling events on the 13th day of the month. The origins of this superstition are debated, with some theories linking it to the number's association with bad luck in ancient Rome and Norse mythology. Additionally, the number 12 is often seen as a symbol of completeness, making 13 feel irregular or incomplete in contrast.