Research: Googling various related terms, Wikipedia, and the archives here.
~Two of my characters are well-known for duelling each other a lot, which means it has to be before guns were popular in duels, because they either would've have to have been crappy shots or one of them would've wound up killing the other (right?). Research has told me guns overtook swords in duelling in the 19th century, but it's probably more like late 18th century, if not earlier.
~These two characters knew each other in school, but they are both wealthy gentlemen, and though I've found out when secondary schools were opened and such, nothing has told me when it became common for the rich to send their kids to fancy schools instead of tutoring them or what not.
I need these two time periods - a time before gun duels and a time when rich kids would've gone to secondary school together - to overlap, and so make my plot make sense. If they don't, I'll change my plot.