source: This area, which later was sometimes called " Prussia proper ", was east of the place where the Vistula River begins.
target: This was the area east of the mouth of the Vistula River, later sometimes called " Pussia Proper . "
references: This was the area east of the mouth of the Vistula River, later sometimes called " Pussia Proper . " This was the area east of the mouth of the Vistula River, sometimes called " Prussia proper . " This is the area of east of the mouth of the Vistula River, later sometimes called " Prussia proper ". This was the area east of the Vistula river, later called " Prussia proper ". This was the area, east of the mouth of the Vistula River, later sometimes called " Prussia proper ". This was the area east of the mouth of the Vistula River, later sometimes called " Prussia proper ". The east mouth of the Vistula River is called " Prussia Proper ".