Zip Codes and Postcodes

In Britain we have postcodes. The postcode for the House of Commons is SW1A 0AA. The SW is the postcode area. (other examples are NN - Northampton area; MK - Milton Keynes area; WS - Walsall area; LE - Leicester area). The 1A is the postcode district within the area. (I live in MK4, I grew up in WS9). Each district is subdivided into sectors (BBC Northampton is in NN1 2). The final two letters are the smallest unit  (known as the postcode unit )- and may represent a single street, part of a street or even an individual building. (The Palace of Westminster has AA for the Commons and PW for the Lords).

Zipcodes in the US are numeric - the basic unit has five numbers - which can cover an area as small as the Senate (20510) and the House of Representatives (20515)

The first three numbers are the Sectional Center Facility - which is the central mail processing facility for that area. The first number represents a number of states (Primary State Prefix), the second and third a region withinn the area designated by the first number (SCF). The final two digits (local zip) represent sorting areas, which may be a city or town, part of a city or town, or a smaller settlement. 

I spent the last week in 20015 and am now in 22308 (can you work out where?). In order to give a more precise location there is "ZIP+4", this may be as small as a city block. The zipcode for Politics and Prose is 20008-2024.