Middlesex ( /ˈmɪdəlsɛks/; abbreviated Middx) was one of the 39 historic counties of England and the second smallest by area. The low-lying county contained the wealthy and politically independent City of London on its southern boundary and was dominated by it from a very early time. The county was significantly affected by the expansion of the metropolitan area of London in both the 18th and 19th centuries, such that from 1855 the south-east was administered as part of the metropolis. When county councils were initially introduced in England in 1889 about 20% of the area of Middlesex, along with a third of its population, was transferred to the County of London, and the remainder formed a smaller county, in the north-west, under the control of Middlesex County Council.
In the interwar years urban London had further expanded, with increasing suburbanisation, improvement and expansion of public transport, and the setting up of new industries outside the inner London area. After the Second World War, the population of the County of London and inner Middlesex was in steady decline, with new population growth only experienced in the outer suburbs. After a Royal Commission on Local Government in Greater London, almost all of the original area was incorporated into an enlarged Greater London in 1965, with small parts transferred to neighbouring Hertfordshire and Surrey. Despite the disappearance of the county council, Middlesex is still used as an area name and was retained as a postal county; which is now an optional component of postal addresses.