Middlesex (/ˈmɪdəlsɛks/ ; abbreviation: Middx) was one of the counties of England until it was abolished in 1965. The county boundaries largely followed three rivers: the Thames in the south, the Lea to the east and the Colne to the west, with a line of hills forming its northern boundary with Hertfordshire. The former area of Middlesex now lies almost entirely in the ceremonial county of Greater London, with smaller parts in the counties of Hertfordshire and Surrey. The county was the second smallest of the historic counties of England, after Rutland.
The name of the county derives from its origin as a homeland for the Middle Saxons in the early Middle Ages, with the county subsequently part of that territory in the ninth or tenth century. The City of London, formerly part of the county, became a self governing county corporate in the twelfth century; the City was still able to exert influence as the sheriffs of London maintained their jurisdiction in Middlesex, though the county otherwise remained separate. To the east of the City, the Tower Division (or Tower Hamlets) had considerable autonomy under its own Lord Lieutenant. To the west, precincts around Westminster and Charing Cross became built up.