Middlesex (, abbreviation: Middx) was an administrative county in southeast England that is now mostly part of Greater London, with small sections in neighbouring counties. It was established in the Anglo-Saxon system from the territory of the Middle Saxons. The historic county included land stretching north of the River Thames from east to west of the City of London with the rivers Colne and Lea and a ridge of hills as the other boundaries. The largely low-lying county, dominated by clay in its north and alluvium on gravel in its south, was the second smallest by area in 1831.