The region that we call “Germany” has seen a lot of iterations over the centuries. Before the 1870s, “Germany” was actually a collection of dozens of different kingdoms, duchies, and principalities. Even since the 1870s, the borders of the nation of Germany have changed vastly over the years. As a result, many of the towns in the border regions have changed hands–sometimes several times. Germany currently borders Denmark, the Netherlands, Belgium, Luxembourg, France, Switzerland, Austria, the Czech Republic, and Poland. As a result, some towns that were in Germany one or two centuries ago, may be within one of these other nations’ borders today, or vice versa.
You may find an old document that lists a person from a German town named “Königsberg in der Neumark,” and when you go to a map, you will find yourself unable to locate the town anywhere. This is because this town is now in Poland and is now called “Chojna”. It can be difficult, at times, to keep track of where certain border towns ended up and how and when their names changed.
One good resource for keeping track of name-changing border towns is Wikipedia’s article on “Cross-border town naming“. The Wikipedia article includes border towns that have changed hands between Germany and the Netherlands, Belgium, Luxembourg, France, Switzerland, Austria, Poland, and the Czech Republic.