ZIP codes in the United Kingdom are called postcodes. They are alphanumeric lines that have five to eight total characters and consist of two parts separated by a space, the outward code and the inward code. Each part in turn is divided into two smaller parts.
The U.K. postcode is hierarchical, zooming in from large areas on the left to smaller units on the right. The first part of the outward code, called the area, consists of one or two letters. As of 2014, there are 124 postal areas in the United Kingdom. For instance, "OX" stands for Oxford, "S" stands for Sheffield and "W" stands for West London. The second part of the outward code is the district, which can be one or two characters long and consist of one or two numbers only or a number followed by a letter. The first part of the inward code is the sector, which consists of the single space and a single number or letter. The last part of the postcode is the unit, which consists of two characters at the end denoting street, address or property.
As of 2014, there are about 1.8 million units consisting of one to 100 individual addresses. Postcodes are not permanent. They change due to shifting distributions of population that occur as a result of building developments and other factors. Old postcodes are often terminated, and new ones are created.