Many western American and Canadian cities have addressing quadrants – NW, NE, SW, SE – to help with their addressing schemes. Some Portlanders use its five quarters as support for its quirkyness. But that’s nothing on London, which takes this to the next level with six quadrants and two parts in the centre.
The UK uses post codes for addressing and locating, in the way Canada feels like it should have. Both countries have alphanumeric codes that are short enough to say and memorize and precise enough to be useful to navigate. But I’ve never seen Canadian postal codes used for anything but mail and locate-closest-thing tools; for route-finding major crossroads are more common, but “Dundas and Spadina” is a fair bit less precise than “NW1 9LJ”. OpenStreetMap will map “V6B 2X6” but no one in Vancouver knows what V6B or V6 covers.
The first part of the postal code is used informally within London as a larger neighbourhood indicator; NW1 means the innermost northwest part. The full code narrows the address down to a few dozen buildings at most — normally a house number and postcode is sufficient to uniquely identify a building.
The usage for neighbourhoods is probably helped by use of major postcodes for disambiguation of the million High Streets and Church Streets within London “postal town.” Most street name signs in Greater London have the first part of the postcode on them to disambiguate.
The actual postcode quadrants are SW, SE, W, NW, N, E, and EC and WC. EC is east central, roughly the City of London, and WC is west central, roughly the West End of London.
The missing quadrants, S and NE, were abolished in the 19th century. The S code was split and merged into SE and SW, and S itself is now owned by Sheffield, which is very much not London. The NE code was merged into E and is now owned by Newcastle, also very much not London.
NW1, SE1, N1 and so on are the innermost districts; but after NW1, numbers are assigned based on alphabetical order of given name of the district, and so NW1 borders NW5 (Kentish Town) and NW8 (St John’s Wood). SE1 borders, clockwise, SE16 (Rotherhithe), SE15 (Peckham), SE5 (Camberwell), SE17 (Walworth), and SE11 (Kennington); SE2 (Abbey Wood) is the farthest SE postcode from SE1.
In east London, the border between E and SE follows the Thames; in west London, the border between W and SW does not. Also, SE1 reaches quite far west and includes all of London’s South Bank, all the way to area across the Thames from the Parliament, and the main station of London and South Western Railway, Waterloo Station.
The postal service reckoned, rightly or wrongly, that postal delivery didn’t have much to do with local government borders, and so postcode borders don’t follow London borders and never have:
… the London postal district, which formed a special post town, did not conform to any administrative boundaries. The postal district was created in 1858 and has periodically been revised. However, at no point has its boundary coincided with either the metropolis (later County of London) of 1855—1965, which was somewhat smaller, or the Greater London area created in 1965, which was much bigger. … Sewardstone, in the Epping Forest district of Essex, is the only locality outside Greater London to be included in the London postal district. — Wikipedia, Postal counties of the United Kingdom