The history of Lancashire begins with its founding in the 12th century. In the Domesday Book of 1086, some of its lands were treated as part of Yorkshire. The land that lay between the Ribble and Mersey, Inter Ripam et Mersam, was included in the returns for Cheshire. When its boundaries were established, it bordered Cumberland, Westmorland, Yorkshire, and Cheshire.
Lancashire emerged as a major commercial and industrial region during the Industrial Revolution. Liverpool and Manchester grew into its largest cities, dominating global trade and the birth of modern industrial capitalism. The county contained several mill towns and the collieries of the Lancashire Coalfield. By the 1830s, approximately 85% of all cotton manufactured worldwide was processed in Lancashire. Accrington, Blackburn, Bolton, Burnley, Bury, Chorley, Colne, Darwen, Manchester, Nelson, Oldham, Preston, Rochdale and Wigan were major cotton mill towns during this time. Blackpool was a centre for tourism for the inhabitants of Lancashire's mill towns, particularly during wakes week.
The county was subject to a significant boundary reform in 1974 that removed Liverpool and Manchester and most of their surrounding conurbations to form the metropolitan counties of Merseyside and Greater Manchester. The detached northern part of Lancashire in the Lake District, including the Furness Peninsula and Cartmel, was merged with Cumberland and Westmorland to form Cumbria. Lancashire lost 709 square miles of land to other counties, about two fifths of its original area, although it did gain some land from the West Riding of Yorkshire.
Today the county borders Cumbria to the north, Greater Manchester and Merseyside to the south, and North and West Yorkshire to the east; with a coastline on the Irish Sea to the west. The county palatine boundaries remain the same as those of the pre-1974 county, with the Duke of Lancaster exercising sovereignty rights, including the appointment of lords lieutenant in Greater Manchester and Merseyside.
The county was established in 1182, later than many other counties. During Roman times the area was part of the Brigantes tribal area in the military zone of Roman Britain. The towns of Manchester, Lancaster, Ribchester, Burrow, Elslack and Castleshaw grew around Roman forts. In the centuries after the Roman withdrawal in 410AD the northern parts of the county probably formed part of the Brythonic kingdom of Rheged, a successor entity to the Brigantes tribe. During the mid-8th century, the area was incorporated into the Anglo-Saxon Kingdom of Northumbria, which became a part of England in the 10th century.