"... instead of 21 'aces' or runs."
"On this date in 1857 it was wisely decided that a baseball game would be made up of nine innings instead of 21 "aces" or runs. The National Association of Baseball Players decided this. They were a group of men in New York and Brooklyn baseball clubs playing under what was known as the "Knickerbocker Rules," and they had just gotten together formally for the first time in January. They had agreed that baseball was "manly and healthful" and should be promoted that way to young men as, they told the paper, an "alternative to billiards ... and other unmentionable night amusements." And they had done away with the practice of hitting the runner with a thrown ball to get him out, which caused fistfights.
But they knew that spectators were coming to baseball games, and under the Knickerbocker Rules a game could be over very quickly. So they changed the rules so as not to disappoint the sport's new fans, which might pay money to see them." (Calendar Writer's Almanac/March 7, 2014)