's map of the County Palatine of Lancaster, 1610
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
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
Kingdom of Northumbria, which became a part of England in the 10th century.
Domesday Book, land between the
Ribble and Mersey were known as "
Inter Ripam et Mersam"
 and included in the returns for
 Although some historians consider this to mean south Lancashire was then part of Cheshire,
 it is by no means certain.
[note 2] It is also claimed that the territory to the north formed part of the
West Riding of Yorkshire.
 It bordered on
Westmorland, Yorkshire, and Cheshire.
The county was divided into
 Lonsdale was further partitioned into Lonsdale North, the detached part north of the sands of
Morecambe Bay including
Cartmel, and Lonsdale South.
Lancashire is smaller than its historical extent following a major reform of local government.
 In 1889, the
administrative county of Lancashire was created, covering the
historical county except for the
county boroughs such as
 The area served by the
Lord-Lieutenant (termed now a
ceremonial county) covered the entirety of the administrative county and the county boroughs, and was expanded whenever boroughs annexed areas in neighbouring counties such as
Wythenshawe in Manchester south of the River Mersey and historically in Cheshire, and southern
Warrington. It did not cover the western part of
Todmorden, where the ancient border between Lancashire and Yorkshire passes through the middle of the town.
During the 20th century, the county became increasingly urbanised, particularly the southern part. To the existing county boroughs of
St. Helens and
Wigan were added
Southport (1905), and
Warrington (1900). The county boroughs also had many boundary extensions. The borders around the Manchester area were particularly complicated, with narrow protrusions of the administrative county between the county boroughs –
urban district formed a detached part of the administrative county, between Oldham county borough and the West Riding of Yorkshire.
census of 1971, the population of Lancashire and its county boroughs had reached 5,129,416, making it the most populous geographic county in the UK.
 The administrative county was also the most populous of its type outside London, with a population of 2,280,359 in 1961. On 1 April 1974, under the
Local Government Act 1972, the administrative county was abolished, as were the county boroughs. The urbanised southern part largely became part of two
 The new county of
Cumbria incorporates the
The boroughs of Liverpool,
St. Helens and
Sefton were included in Merseyside. In Greater Manchester the successor boroughs were
Trafford (part) and
Wigan. Warrington and
Widnes, south of the new Merseyside/Greater Manchester border were added to the new non-metropolitan county of
urban districts of
Bowland Rural District and the parishes of
Bracewell and Brogden and
Skipton Rural District in the
West Riding of Yorkshire became part of the new Lancashire.
 One parish,
Simonswood, was transferred from the borough of
Knowsley in Merseyside to the district of
West Lancashire in 1994.
 In 1998 Blackpool and
Blackburn with Darwen became independent
Wars of the Roses tradition continued with Lancaster using the red rose symbol and York the white. Pressure groups, including
Friends of Real Lancashire and the
Association of British Counties advocate the use of the historical boundaries of Lancashire for ceremonial and cultural purposes.