The subject under discussion is the place where in 1243 or 1244 the Teutonic army was defeated – the basin called “Rensen”, “Rense”, or “Reußen”, “Reussenn”, “Reüßen”, “Reusen”. The names enumerated above come from the manuscripts of the chronicle of Peter from Dusburg – written in its major part before 1326, and continued until 1330. It is the oldest source which gives the name of the site of the battle and information about it. The form “Rensen”, appearing in literature, was identified with Rządz (Rządzkie Lake) near Grudziądz. Jarosław Wenta recognizing first the form “Rensen” and later also “Reusen” stated that the place must have been situated near Chełmno. As the chronicle of Peter from Dusburg failed to provide explicit identification, we employed historiographical monuments which used the manuscripts of the chronicle, as well as other sources giving the knowledge about the name we are interested in. The majority of the sources contain the name “Rensen” and names similar to it, which are identified with Rządzkie Lake. Moreover, the cartographic sources including Rządzkie Lake near Grudziądz did not recognize the basin “Reussen” near Chełmno. It is also known that the diphthong “eu” did not appear in the written German language in Prussia until the turn of the 14th and 15th centuries. Peter from Dusburg did not use the diphthong “eu” in German words denoting proper names. | us, it is very unlikely that he used the form “Reussen” or a form similar to it. The author further explains his opinion on the existence of such forms in the manuscripts. In Latin palaeography the lower-case letter “n” resembled “u”. The copyists, not knowing the geography of Prussia, must have made a palaeographic mistake changing “n” into “u”. To sum up, it is quite certain that the basin “Reussen” (“Reusen”) near Chełmno never existed, and the battle took place in Rządz near Grudziądz.