Middlesex is one of the historic counties of England. 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. Its former area now lies almost entirely in the ceremonial county of Greater London, with smaller parts in the counties of Hertfordshire and Surrey. The county is the second smallest of the historic counties of England, after Rutland.