Northumberland occupies the extreme north-east of England, bounded by the North Sea to the east, the bishopric of Durham and Cumberland to the south, and Scotland to the north and west, separated from it by the River Tweed. Eighteenth-century surveys measured it at about sixty-five miles long and forty-five broad, some 1,370,000 acres divided among twelve market towns and 460 parishes, with a population of over 126,000 lying within the diocese of Durham. Lying between two seas at one of the narrowest points of England, the county's climate was found milder than its northerly latitude suggested, its inhabitants noted for long life and good health despite centuries spent on a contested military frontier.
The fertile eastern lowlands produced good wheat and rich riverside meadow, while the western uplands, rising toward the Cheviot Hills, were largely barren moorland, their value lying instead beneath the surface: the south-east of the county was already, by the 1770s, shipping some 600,000 chaldrons of coal a year from the Tyne to London, alongside substantial quantities of lead, in a trade that laid the foundation for Newcastle's later industrial importance. The county's rivers, chief among them the Tyne, Tweed and Coquet, yielded abundant salmon and trout, and Northumberland gave its ducal title to the ancient and powerful Percy family, long its dominant landowners.
Few English counties carry so dense a legacy of border warfare in their antiquities: Alnwick Castle, seat of the Percys; Warkworth, Dunstanburgh and Bamburgh castles along the coast; Norham Castle on the Tweed; and Hadrian's Wall itself, still traceable across the county's western moors, all testify to centuries of conflict with Scotland. Holy Island, with its ruined priory founded by St Aidan in the seventh century as a cradle of English Christianity, and the monastic remains at Hexham and Tynemouth completed a landscape shaped, more than almost any other English county, by its position on the edge of the kingdom.