How common were AL to NL Trades?
 

Just a background question for you history buffs, to inform a dynasty idea that I'm working on.

So, I know that even after the establishment of the World Series and the creation of the office of the Commissioner of Baseball, the two leagues were really thought of as fairly separate institutions well into the 20th century. What I'm curious about, then, is how long it took for trades between AL and NL teams to become commonplace. Would a cross-league trade have been noteworthy in, say, 1950?

There would have been no trades between the AL and NL during their "war" (1901-03). After that, I believe, in order for an interleague trade to happen, the players being swapped had to clear waivers in their own leagues first. That tended to cut down on the number of interleague player trades. Instead, it appears that most transactions were players-for-cash. You can look at team-to-team transactions at BR. Those waiver rules were lifted some time in the 1950s.