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: