It's cool that the Slavic "circle" via the Old Slavic "krug" and the English "ranger/ring" via the Old German "hringaz" came from the Proto-Indo-European "kerang/ker" meaning "something round". Then, through a series of questioning of the terms "line", the word "ranger" on the American frontier took on the final meaning of "one who guards the borders" (although in Britain it was literally "foresters").