They were status-Quo in medieval Europe particularily among the Nobility.l The unwritten rule was nobody was to marry outside their caste or social ranking.In some ways, this backed up the custom. Joe Doaks the farmer could not come near Princess , shall we say Anastasia. In the armed forces- marriages could only be between officers of the same rank category- For example a Lieutenant could not Marry a Sergeant ( one party would have to be a Wac, Wave, or equivalent, we are not considering homosexual unions here. This restriction on social partners has a sort of un-American ring, and outside of the Armed Forces , it is not practiced. Of course in some countries, there was a flip side, a Woman who ( civil here) would marry a Field Grade officer- such as a Major, would automatically become a Contessa! Not a bad idea, never would work in US, as we do not have titled nobility.