The phrase “walk-off home run” didn’t apply before the 1920 season.

If a player hit a home run in the bottom of the ninth inning or in the bottom of an extra inning before 1920, the home run was actually only credited as a single, double or triple, depending on how many bases it took to advance the winning run.

For instance, if the score was tied in the bottom of ninth with a runner on third, if the batter hit a home run, he would only be credited with a single.

Another fun Fact