Originally Cölln was the sister town of old Berlin located on the southern Spree island in Germany's Mark Brandenburg of the 12th century. Today this twin city zone is located in the center of modern Berlin in the vicinity of the Museum Island.

With the date October 28th, 1237 Cölln is first documentary mentioned, Berlin in 1244. Both towns were only separated by the river Spree, hence there were cooperations right from the start. The trade road from Magdeburg to the east crosses the twin town and the inland water-transport also ended there so that Cölln/Berlin came fast to prosperity.

A common policy of Berlin and Cölln led 1308 to a first alliance with other towns in the Mark to preserve their rights against the sovereign and for defense. The Elector Friedrich II ends the autonomy of Cölln/Berlin and declares the twin town to his residence in 1451. In 1709 the twin cities Cölln and old Berlin merged together to one real city with one administration which was the base of the modern Berlin of today. Since old Berlin was twice as big as Cölln the merged city is called Berlin.