Originally there were ten districts - WC, EC and the eight compass points. Each stretched out approximately 12 miles from the central post office (near St Paul's Cathedral) [map]
SW was, and remains, the only London postal district to cross the Thames.
In 1866 Anthony Trollope merged the NE and E districts under E. He also nudged in the outer boundary, removing Ilford, Romford and Dagenham from the system. NE would later be used for Newcastle.
In 1868 he abolished the S district by splitting it between SW and SE. S would later be used for Sheffield. [map]
In 1917, to improve wartime efficiency, the eight postal districts were subdivided and numbered. The head post office in each district was coded 1, then the remaining areas were numbered in alphabetical order of the main delivery office. [map]
Because W1 (the West End) was so important, the western head district (in Paddington) got to be W2 instead.
SW had two head districts (inner southwestern and outer southwestern) so had two alphabetical lists, one for SW1–SW10 and another (starting in Battersea) for SW11–SW20.
SE also comprised two alphabetical lists, the inner district being SE1-SE18 and the outer district being SE19-SE27.
Multi-character postcodes were introduced across London in the 1970s, each starting with the three or four characters of the existing district code.
Ten postcode areas have been further subdivided by adding another letter on the end. These are E1, N1, W1, EC1, EC2, EC3, EC4, WC1, WC2 and SW1.
New code SE28 was carved out of SE2 and SE18 when Thamesmead was built. The only other fresh code is E20, created in 2011 for the Olympic Park, and formerly part of E15. There is no E19.
The only London boroughs entirely covered by "compass point" postcodes are Camden, Hackney, Hammersmith and Fulham, Haringey, Islington, Kensington and Chelsea, Southwark, Tower Hamlets, Wandsworth and Westminster. Meanwhile Barking and Dagenham, Havering, Hillingdon and Sutton lie entirely outside the London postal district.
Sewardstone, an Essex village north of Chingford, is the only place outside the Greater London boundary but within the London postal district.
The outer London postal region includes postcodes AL, BR, CR, DA, EN, HA, IG, KT, RM, SM, TW, UB and WD.
If the NE postcode still existed, it would contain modern postcodes E2, E4, E5, E8, E9, E10, E11, E17 and E18. An NE street sign remains on Victoria Park Road in Hackney.
If the S postcode still existed, it would contain modern postcodes SW2, SW4, SW9, SW12, SW16 and SW17, plus SE5, SE11, SE17, SE19, SE21, SE24, SE24, SE25 and SE27 (I think).