Four semicircular canals — Singel, Herengracht, Keizersgracht, Prinsengracht — dug in the 17th century around the old town and lined with narrow merchant houses, step gables and the hoisting beams still bolted under the roofs. Walk the quays or take a boat under the bridges; after dark seventy lit bridges double themselves in the still water. The crossing at Reguliersgracht is where everyone stops to photograph.