Humans and computers have both been struggling since then to try to get them right. UK Postcodes were designed to be easier for humans to understand and remember but their mix of numbers and letters makes them difficult for computers. Most of the rest of the world uses ZIP codes which, being entirely numeric, are much easier for computers to understand.
Area codes have one or two letters.
District codes usually have one or two digits but, in some areas of London, they are one digit and a letter. Sector codes are always a single digit and Walk codes are always two letters. These rules mean that a postcode will have one of four possible patterns for Area, District, Sector and Walk AD SWW ADD SWW AAD SWW AADD SWW Area codes are letters, districts are numbers, sector is always a number and the walk code is always two letters. An added complication is that 2nd character of district code could be a letter. All letters are in upper case and, to be strictly accurate, the letters C, I, K, M, O and V are not allowed in the walk code.