Capital and largest town of the Isle of Man, on the east coast at the mouth of the rivers Dhoo and Glass.

The Manx form Doolish was explained by an early dictionary as referring to the meeting of the two rivers, one described as black and the other as green or white, their junction producing a dark green colour. The rivers meet immediately above the harbour and provided the anchorage around which the old town developed.

Douglas was still a relatively small settlement in the early eighteenth century, but its harbour and position on the east coast encouraged rapid commercial growth. George Moore, the Peel merchant, began acquiring property there in the 1750s. In 1751 he complained that houses and cellars were difficult to obtain because the town was crowded with people coming in from elsewhere and almost every available space was occupied.

The harbour became increasingly important as trade expanded. By the end of the eighteenth century harbour duties, together with the herring custom and bay fisheries, formed a substantial part of the revenue available for the maintenance of the Island's ports. The harbour fund for the year ending July 1791 produced £319 5s. 1½d. in harbour duties and £95 19s. 3½d. from the herring custom and bay fisheries, although £327 was spent on repairs.

Douglas gradually displaced Castletown as the Island's principal commercial centre. The growth of the town brought new churches, streets, quays and public buildings, and the settlement spread beyond the older harbour-side streets into Braddan and Onchan. The old river crossings and the harbour continued to determine the shape of the centre.

Municipal growth produced increasingly expensive public works. In 1929 the House of Keys was reminded that Douglas Corporation, while still a Board of Commissioners, had paid £140,000 for the Douglas Waterworks Company and subsequently spent between £180,000 and £190,000 developing the undertaking.

By the twentieth century the House of Keys and Tynwald were also meeting in Douglas, although the Island's older administrative centre remained at Castletown for much of the nineteenth century.