The East Prussian city of Königsberg (now part of Russia and known as Kaliningrad) straddles both banks of the River Pregel and an island, called Kneiphof, located right where the river divides into two branches. In the 18th century, seven bridges spanned various segments of the river, connecting different parts of the city.
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![](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2006/09/6155.jpg?resize=126%2C150&ssl=1)
These bridges were the subject of a well-known puzzle at the time: Could a person follow a path through the city that crosses each of the bridges only once, then returns to the starting point?