Most of London was originally part of the county of Middlesex, but London gradually gobbled it up, and and nearly all of what was once Middlesex is now in London.
The Middle Saxons may once have had their own kingdom of Middlesex, but if so, it didn’t last for long. Their territory may even have crossed the Water to include Surrey and what is now South London. But by the seventh century, Middlesex was just a province of the kingdom of Essex, entirely north of the Thames, the River Lea forming its boundary with Essex proper, while its western border was the River Colne, which still separates the Borough of Hillingdon from the county of Buckinghamshire to this day.
The symbol of Middlesex is a trio of notched short swords called saexes. They were the trademark weapon of Saxon warriors, and it was apparently from them that the Saxons got their name. The saexes on Middlesex’s (and also Essex’s) flag and coat of arms are not historically accurate, and look more like Arabic scimitars, possibly inspired by ones encountered during the Crusades. By the Middle Ages, Middlesex was already dominated by London. At first the county was made up largely of farms supplying food to the city, but gradually London’s population spilled out beyond the city’s walls, and more and more parts of Middlesex became suburbs of London.
Before 1832, the voting qualification in county constituencies was a forty-shilling freehold, which meant that to vote, you had to own land worth at least £2. When it was introduced in 1430, that was a lot of land, but by the mid-eighteenth century, quite a few Middlesex homeowners qualified to vote on that basis, and the county had a large and sophisticated electorate of some 3,500–4,000 people. In 1769, they returned as their MP the radical journalist John Wilkes, who’d previously been prosecuted and expelled from parliament for crimes such as sedition and publishing a rude poem. Unamused with the Middlesex result, parliament voided the poll and called a by-election. Wilkes won it unopposed. Parliament voided it again and called another by-election. Wilkes won it unopposed again. Finally, the Tories put up a candidate against him. Wilkes won by 1,143 votes to 296, but parliament declared the Tory elected. He served as a much-hated local MP until the next general election, in 1774, at which Wilkes was elected yet again, and parliament finally accepted the result and let him take his seat.
In 1899, the County of London was created. It expanded beyond the City to take in a fifth of Middlesex’s land and a third of its population. The West End and the East End became part of London, but Middlesex remained as a halo around London’s north and west, stretching from Staines round to Enfield. Increasingly however, it was not so much an English county as an outer part of London, and in 1965 that reality was officially recognized, and Middlesex was incorporated into Greater London, shedding Potters Bar, Staines and Sunbury in the process.
Even after the County of London was created in 1899, encompassing Westminster, Middlesex County Council still met at the Middlesex Guildhall on the west side of Parliament Square in Westminster, now the home of the UK’s Supreme Court. Even then, some areas of Middlesex were still rural, but because most parts of the county had become suburbs of London, many had north, west or northwest London postcodes, such as Tottenham N17, Ealing W5, or Hendon NW4.
But other parts of the county, including Hounslow, Wembley, Edgware and Enfield, kept Middlesex addresses until 1996. Even now they have their own postcodes: EN for Enfield, HA for Harrow, UB for Uxbridge and TW for Twickenham. and some people in those places still put “Middx” in their addresses to this day, although it’s officially now redundant. Oddly, Enfield is six miles adrift from the rest of the postal county, cut off from it by Barnet, which the post office still considered part of Hertfordshire. A county of Middlesex does still survive, but it’s in New Jersey. Meanwhile, London’s Middlesex lives on in various ways, with a university, a cricket club, and an annual day on 16 May. unfortunately however, Jeffrey Eugenides’s 2002 novel Middlesex is not about Middlesex, and isn't even set there.