Hajnal line
According to Wikipedia:
“The Hajnal line is a border that links Saint Petersburg, Russia and Trieste, Italy. In 1965, John Hajnal discovered it divides Europe into two areas characterized by different levels of nuptiality. To the west of the line, marriage rates and thus fertility were comparatively low and a significant minority of women married late or remained single; to the east of the line and in the Mediterranean and select pockets of Northwestern Europe, early marriage was the norm and high fertility was countered by high mortality.”
But really I’m posting this mostly because I like the phrase “different levels of nuptiality.”