Sanquhar (Scottish Gaelic An t-Seann Chathair: 'the Old Fort') is a town in the south of Scotland in Dumfries and Galloway, on the River Nith. It lies north of Thornhill and west of Moffat. It is a Royal Burgh.
Sanquhar is notable for its tiny post office (established in 1712), claimed to be the oldest working post office in the world.
More info can be found on the Sanquhar website.