source: This was the area east of the mouth of the Vistula River, later sometimes called "Prussia proper".
target: Prussia proper was the area east of the mouth of the Vistual River.
references: Prussia proper was the area east of the mouth of the Vistual River. The area was east of the beginning of the Vistula River. Later on, some people called it "Prussia proper." Prussia proper was the place east of the mouth of the Vistula River. The area east of the mouth of the Vistula River was sometimes called "Prussia proper". This area east of the mouth of the Vistula River was later sometimes called "Prussia proper". This was the area which is east of the opening of the Vistula River and later people sometimes called it "Prussia proper". This was east of the mouth of Vistula River, sometimes called "Prussia proper." This was the location east of the mouth of the Vistula River, later sometimes called "Prussia proper." The area east of the mouth of the Vistula River is sometimes called Prussia proper. Prussia proper was another name for the eastern mouth of the Vistuala River.