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They demarcated the 18th century border between Poland (Warmia) and Prussia-Brandenburg:
On one side of the border the population was Lutheran and on the other side Roman Catholic:
https://www.theapricity.com/forum/sh...many-by-county
Warmia in red, surrounded by Ducal Prussia:
https://upload.wikimedia.org/wikiped...%84skieIRP.png
Triangular-shaped Catholic area in East Prussia:
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