Middlesex was one of the 39 traditional Counties of England. It includes the City of London, which was self-governing from the thirteenth century. London's northwestern suburbs steadily covered large parts of Middlesex, especially following the coming of the railways.
The administrative and historic boundaries around Middlesex diverged very early on, with the recognition of the City of London as an independent county borough. In 1888 much of the area became part of the County of London.
Middlesex as a traditional county still technically exists, however, although it appears on no modern maps and it is certainly still sometimes used in postal addresses.