Baseball is a sport that has seldom altered its rules, although recent years have seen a number of changes implemented in the major leagues:
1969: After the Year of the Pitcher, when Denny McLain won 31 games and Bob Gibson posted a 1.12 ERA in 1968, MLB lowers the height of the mound from 15 to 10 inches.
1973: With offense sagging again, the American League adopts the designated hitter rule for what was originally approved as a three-year trial.
2008: MLB adds limited instant replay for home run calls. In 2014, replay expands to include fair/foul calls, catch/trap calls, fan interference and ground-rule doubles, among other rulings.
2014: Three years after All-Star catcher Buster Posey breaks his leg, MLB minimizes home-plate collisions by guaranteeing the runner a clear path to the plate unless the catcher already has the ball.
2016: Takeout slides designed to break up double plays are reined in, with runners needing to make a “bona fide” attempt to “reach and remain” on second base.
2017: The no-pitch intentional walk rule is instituted to save time.