Greater Manchester spans 493 square miles (1,277 km2), which roughly covers the territory of the Greater Manchester Built-up Area, the second most populous urban area in the UK. It is landlocked and borders Cheshire (to the south-west and south), Derbyshire (to the south-east), West Yorkshire (to the north-east), Lancashire (to the north) and Merseyside (to the west). There is a mix of high-density urban areas, suburbs, semi-rural and rural locations in Greater Manchester, but land use is mostly urban — the product of concentric urbanisation and industrialisation which occurred mostly during the 19th century when the region flourished as the global centre of the cotton industry. It has a focused central business district, formed by Manchester city centre and the adjoining parts of Salford and Trafford, but Greater Manchester is also a polycentric county with ten metropolitan districts, each of which has at least one major town centre and outlying suburbs.
For the 12 years following 1974 the county had a two-tier system of local government; district councils shared power with the Greater Manchester County Council. The county council was abolished in 1986, and so its districts (the metropolitan boroughs) effectively became unitary authority areas. However, the metropolitan county has continued to exist in law and as a geographic frame of reference, and as a ceremonial county, has a Lord Lieutenant and a High Sheriff. Several county-wide services were co-ordinated through the Association of Greater Manchester Authorities until April 2011, when the Greater Manchester Combined Authority was established as the strategic county-wide authority for Greater Manchester, taking on functions and responsibilities for economic development, regeneration and transport. A further devolution of powers to Greater Manchester took place upon the election Andy Burnham as the inaugural Mayor of Greater Manchester on 4 May 2017
Before the creation of the metropolitan county, the name SELNEC was used for the area, from the initials of "South East Lancashire North East Cheshire". Greater Manchester is an amalgamation of 70 former local government districts from the former administrative counties of Lancashire, Cheshire, the West Riding of Yorkshire and eight independent county boroughs. Since deindustrialisation in the mid-20th century, Greater Manchester has become known as an exporter of media and digital content, for its guitar and dance music and football clubs.
Although the modern county of Greater Manchester was not created until 1974, the history of its constituent settlements goes back centuries. There is evidence of Iron Age habitation, particularly at Mellor, and Celtic activity in a settlement named Chochion, believed to have been an area of Wigan settled by the Brigantes. Stretford was also part of the land believed to have been occupied by the Celtic Brigantes tribe, and lay on their border with the Cornovii on the southern side of the River Mersey. The remains of 1st-century forts at Castlefield in Manchester, and Castleshaw Roman fort in Saddleworth, are evidence of Roman occupation. Much of the region was omitted from the Domesday Book of 1086; Redhead states that this was because only a partial survey was taken, rather than sparsity of population.
During the Middle Ages, much of what became Greater Manchester lay within the hundred of Salfordshire – an ancient division of the county of Lancashire. Salfordshire encompassed several parishes and townships, some of which, like Rochdale, were important market towns and centres of England's woollen trade. The development of what became Greater Manchester is attributed to a shared tradition of domestic flannel and fustian cloth production, which encouraged a system of cross-regional trade. In the late-18th century, the Industrial Revolution transformed the local domestic system; mechanisation enabled the industrialisation of the region's textile trade, triggering rapid growth in the cotton industry and expansion in ancillary trades. Infrastructure such as rows of terraced housing, factories and roads were constructed to house labour, transport goods, and produce cotton goods on an industrial scale for a global market. The townships in and around Manchester began expanding "at an astonishing rate" around the turn of the 19th century as part of a process of unplanned urbanisation brought on by a boom in industrial textile production and processing. This population increase resulted in the "vigorous concentric growth" of a conurbation between Manchester and an arc of surrounding mill towns, formed from a steady accretion of houses, factories and transport infrastructure. Places such as Bury, Oldham and Bolton played a central economic role nationally, and by the end of the 19th century had become some of the most important and productive cotton-producing towns in the world. However, it was Manchester that was the most populous settlement, a major city, the world's largest marketplace for cotton goods, and the natural centre of its region. By 1835 "Manchester was without challenge the first and greatest industrial city in the world"; and by 1848 urban sprawl had fused the city to its surrounding towns and hinterland to form a single continuous conurbation. In the 1910s, local government reforms to administer this conurbation as a single entity were proposed.