Because a pitcher’s motion is so strenuous on the muscles of the arm and shoulder, it is not recommended that a pitcher throw on consecutive days. Therefore, managers employ the use of a starting rotation, or a list of their five best pitchers, in order, who pitch during five consecutive games. This allows the starting pitcher from game one to have four to five days of rest before throwing hard again.

To be clear, only one pitcher from the starting rotation throws on any given day. The starting rotation is composed of several starting pitchers who pitch on consecutive days from each other. When one of the starting pitchers from the starting rotation pitches a game, he throws until the coach takes him out of the game. At that point, the starting pitcher is replaced not with another starting pitcher from the starting rotation, but instead with a relief pitcher from the bullpen.

The starting rotation is the linchpin of any good team. If the starting rotation is strong throughout, that team is likely to be more successful than a team with a weaker starting rotation or only a mediocre one. The first pitcher in the starting rotation is typically the best pitcher on the team, or what is known in baseball circles as the Ace. The Ace pitches the first game in the rotation, which is why most opening day games are pitched by the Ace.

The order of pitchers in the starting rotation descends from there. The fifth spot in a rotation usually goes to the least experienced or least talented of the pitchers. It is not uncommon to see the fifth position pitcher changed with another pitcher as the season progresses. Also, if any of the pitchers in the starting rotation get injured throughout the course of the season, the starting rotation can change as new pitchers are added and taken away. While there is no set rule as to how many pitchers make up the starting rotation, four to five pitchers in rotation at any time is most common.