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 commercial tenants who fear superstitious customers.
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Also from a certain date in the uk you'lll find that new estates, roads etc dont have a number 13. It's a luck thing, and also they believed people wouldn't want to buy a number 13 house.