The City of London is also known as "The Square Mile" and is as its nickname implies approximately one square mile in area (about 2.5 square kilometres). Originally enclosed by a city wall, it is now the financial district of modern London, but retains many of its historic privileges such as its own Lord Mayor and separate police force.

Historically the Roman city of Londinium was founded here, and for hundreds of years the City has been the country's centre of trade and commerce. Expansion outside the original square mile started to take place in earnest during the 18th century and by the 19th century the original City was only a small part of the wider metropolis. Most of this wider area was north of the River Thames and formed part of the county of Middlesex. The expansion south of the river formed part of the counties of Surrey (towards the south and southwest) and Kent (towards the southeast).

In 1832, seven Parliamentary boroughs were created in addition to the City of London - Finsbury, Greenwich, Lambeth, Marylebone, Southwark, Tower Hamlets and Westminster. These remained in place until the County of London was formed in 1900, which consisted 28 London boroughs , 12 of which were designated as 'Inner' London boroughs and the remaining 18 as 'Outer' London. The County of London existed until 1965, when it became Greater London, along with most of Middlesex. Greater London itself was abolished as an administrative area in 1986 when its functions were given to the 32 London boroughs, although the term 'Greater London' is still used to describe the boroughs collectively. Th number of London boroughs expanded as parts of the Home Counties were absorbed into the greater metropolitan area and become part of London. Most of this reorganisation has taken place since the 1960s, so it has not affected the location of historic records dating from the 19th century and earlier which remain in the relevant county record offices.

The population of the City of London fell rapidly during the 19th century as people moved out towards the suburbs, and today only a few thousand people live within the City of London itself.

Records of both the City and County of London are held at the London Archives (previously known as the London Metropolitan Archives) in the City of London, with some records also held by local London borough archives.

The map below shows the City of London in about 1830. Click on the map for a larger version.

The map below shows the County of London in 1900 and its divisions into the various boroughs. Click on the map for a larger version.

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