Question: What is the rule that makes a major difference between the National League and the American League that allows another player to bat for the pitcher?

The paixiao (traditional: 排簫; simplified: 排箫; pinyin: "páixiāo"; also pái xiāo, pai-hsiao, pronounced ] ) is an ancient Chinese wind instrument, a form of Pan flute.  A major difference between the Chinese Paixiao and the panpipes used in European and South American traditions, is that at the top of the Chinese instrument the pipe holes are each cut angled or with notches.  This allows for bending the pitch in similar capacity to the dongxiao down a minor second.  This allows Chinese paixiao to be fully chromatic without loss in timbre, even though the included pipes are tuned diatonically. <SEP> The History of the American League, officially known as the American League of Professional Baseball Clubs and often abbreviated AL, stretched back into the late-19th century.  The American League has one notable difference over the National League, and that is the designated hitter rule.  Prior to 2000, when the AL and NL were dissolved as separate entities and merged into the organization called Major League Baseball, the American League was one of the two leagues that made up major league baseball.  Originally a minor league known as the Western League, the league later developed into a major league after the American Association disbanded.  In its early history, the Western League struggled until 1894, when Ban Johnson became the president of the league.  Johnson led the Western League into major league status and soon became the president of the newly renamed American League.  Under the rule, a team may use a batter in their lineup who is not in the field defensively, compared to the old rule that made it mandatory for the pitcher to hit. <SEP> A defensive substitution in the game of baseball occurs when a currently non-playing player is placed into the field in place of another player, typically due either to injury or the appearance of a pinch hitter.  In the American League, a pinch hitter is usually substituted for the position player in whose place he batted (although another common substitution is pinch-hitting a utility infielder for a second baseman and subsequently switching him for the shortstop in the field while moving the shortstop over to the now-vacant second base position).  In the National League, which does not use the designated hitter rule, pinch hitters sometimes bat for pitchers. However, another common pitcher-substitution strategy in the NL is the double switch, in which a pitching change is coupled with a pinch-hitter who is also defensively substituted for a second player. <SEP>  The rule allows teams to have one player, known as the designated hitter (abbreviated DH), to bat in place of the pitcher.  Since 1973, most collegiate, amateur, and professional leagues have adopted the rule or some variant.  MLB's National League and Nippon Professional Baseball's Central League are the most prominent professional leagues that do not use a designated hitter. In baseball, the designated hitter rule is the common name for Major League Baseball Rule 5.11, adopted by the American League in 1973.