What did the Treaty of Prague say about a possible vote in North Schleswig?

The Treaty of Prague (1866), which concluded the Seven Weeks' War, included a provision stating that the districts of North Schleswig would be reunited with Denmark if their inhabitants expressed a wish to be so by a vote (plebiscite) of the majority.1

However, this provision was never implemented; in 1878, Prussia and Austria reached an agreement to cancel it. It was not until after Germany's defeat in World War I that plebiscites were finally held in 1920 to determine the final disposition of the territory.21