This postcard shows the churches of Frankfurt (Oder) in Germany. There are 2 cities called Frankfurt in Germany. The bigger (and better known) is situated in the Western part of Germany and is called Frankfurt am Main. Frankfurt (Oder) is much smaller and actually half of this city is in Poland and is called Słubice :) The border is on the Oder (or Odra in Polish) river.
There are more cities like that on the German-Polish border, because of the decision that the new border between these countries (after World War 2) should be mainly on 2 rivers - Nysa Łużycka (Lausitzer Neisse) and Odra (Oder). Now, when both Poland and Germany are members of EU, it's quite fun to visit a city, which is in 2 different country and where you can always go abroad for a walk or to cafe :)