Some buildings don't have a 13th floor because the number 13 is often considered unlucky. This superstition leads the architect or builder to skip this number and go straight from floor 12 to floor 14.
The number 13 has been considered unlucky for centuries, and numerous theories explore why this superstition might exist. One is that the ancient Sumerians considered the number 12 to be the perfect number. Another consideration is that both the Bible and ancient Norse mythology contain stories about a 13th guest showing up to a party and causing havoc. In the Bible it was Judas, the man who betrayed Jesus; for the Norse, it was the god Loki.