The levels of a multi-story building are numbered sequentially, from "one" or "ground" upwards. In some countries, the number 13 is considered unlucky and building owners will sometimes purposely omit the thirteenth floor. Even landlords who are not themselves superstitious realize that the rentability of suites on the 13th floor might be compromised because of superstitious tenants, or tenants who are afraid their own customers might be superstitious.