The thirteenth floor is a designation of a level of a multi-level building that is often omitted in countries where the number 13 isconsidered unlucky. Omitting the 13th floor may take a variety of forms; the most common include denoting what would otherwise be considered the thirteenth floor as level 14, giving the thirteenth floor an alternate designation such as 12A or M (the thirteenth letter of the Latin alphabet), or closing the 13th floor to public occupancy or access (e.g., by designating it as a mechanical floor). Reasons for omitting a thirteenth floor include triskaidekaphobia on the part of the building's owner or builder, or a desire by the building owner or landlord to prevent problems that may arise with superstitious tenants, occupants, or customers. Based on an internal review of records, Dilip Rangnekar of Otis Elevators estimates that 85% of the buildings with elevators did not have a floor named the 13th floor. Future building designers, fearing a fire on the 13th floor, or fearing tenants' superstitions about the rumor, decided to omit having a 13th floor listed on their elevator numbering. This practice became commonplace, and eventually found its way into mainstream culture and building design.
Photo by: Filip Friščić (Hotel Okura, Amsterdam)
Source: Wikipedia