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The term "starting rotation" refers to the group of pitchers who are used by a baseball team to start games. They typically take turns starting games, following a certain order, and then start over at the beginning of the order, or rotation. In modern professional baseball, a team's starting rotation almost always consists of five pitchers, although a four-pitcher rotation might be used temporarily. The team's best starting pitcher — or "ace" — usually is first in the rotation. Its second-best starter usually is second in the order, and so forth through the rest of the starting rotation, although this is not always the case, for various reasons.
Starting rotations are needed because the act of pitching a baseball can be very strenuous on the muscles of the arm and shoulder. A pitcher who has thrown a certain number of pitches or innings in a game usually must have several days to rest before being able to effectively pitch again. For example, many starting pitchers rest for four days before pitching in another game, although these rest days can include various forms of exercise and even some throwing. A pitcher who is used too much without enough rest between starts can be more susceptible to fatigue and injury.
Most baseball teams consider their starting rotations to be among the most important aspects of the team. This is because starting pitchers typically are asked to pitch more than half of the innings in games that they start, or even the majority of the innings. Some are able to pitch entire games, although this has become increasingly rare because the use of relievers — players who come in to replace previous pitchers — has become more widespread, mostly for strategic reasons. Pitchers who are part of their teams' starting rotations are almost never used as relievers, except in extreme circumstances.
The order of pitchers in a starting rotation is determined by the team's manager and almost always begins with the team's best starter. From there, the rotation might continue with the second-best pitcher, then the third-best, and so forth, or the manager might use a certain strategy to set the rotation. For example, some managers prefer to alternate right-handed pitchers and left-handed pitchers, if possible. Others might alternate different styles of pitchers, such as having a hard thrower followed by a pitcher who is known for throwing slow curveballs or changeups.
Even professional teams don't have games scheduled for every day during the baseball season, and some games are postponed because of rain. Having a day without a game can leave a manager with a decision to make about the team's starting rotation, such as whether to skip the pitcher whose turn it would have been or to push everyone in the rotation back by one day. Pitchers who are lower in the rotation sometimes will be skipped so that the team's best starters — those at the top of the rotation — will be able to pitch in more games. Sometimes, a manager will change the order of the starting rotation or replace a pitcher in the rotation, such as when one of the starters is injured or has not pitched well enough.