A grens is a border.
Municipal borders determined where a couple would register the birth of their child, where they were taxed, where they would apply for a permit. Judicial borders determined which court prosecuted a ciminal, granted a divorce, or appointed a guardian.
Borders affected our ancestors’ lives. My ancestor Arend Kastein moved across the street and tried to apply for a job as a policeman. But since that street happened to be the Dutch-German border, he needed to be naturalized first.