The northern part of Brookline, roughly north of the D-line tracks, is urban in character, as highly walkable and transit rich. The population density of this northern part of town is nearly , similar to the densest neighborhoods in nearby Cambridge, Somerville and Chelsea, Massachusetts (the densest cities in New England), and slightly lower than that of central Boston's residential districts (Back Bay, South End, Fenway, etc.). The overall density of Brookline, which also includes suburban districts and grand estates south of the D-line, is still higher than that of many of the largest cities in the United States, especially in the South and West. Brookline borders Newton (part of Middlesex County) to the west and Boston (part of Suffolk County) in all other directions; it is therefore non-contiguous with any other part of Norfolk County. Brookline became an exclave of Norfolk County in 1873, when the neighboring town of West Roxbury was annexed by Boston (and left Norfolk County to join Suffolk County). Brookline refused to be annexed by Boston after the Boston–Brookline annexation debate of 1873.