A starting pitcher (also referred to as the starter) is the pitcher who delivers the first pitch to the first batter of a game. A pitcher who enters the game after the first pitch of the game is a relief pitcher.
Under ideal circumstances, a manager of a baseball team would prefer a starting pitcher to pitch as many innings as possible in a game. Most regular starting pitchers pitch for at least five innings on a regular basis, and if a pitcher is unable to do so, there is a high probability that he will, in the future, be relegated to duty in the bullpen. In modern baseball, a starting pitcher is rarely expected to pitch for more than seven or eight innings, at which point, responsibility for the game is passed to specialist pitchers, — set-up pitchers and closers.
Often, a starting pitcher is subject to a pitch count, meaning the manager will remove him from the game once he has thrown a specific number of pitches. The most common pitch count for a modern pitcher is in the neighborhood of one hundred. Pitch counts are especially common for starting pitchers who are recovering from injury. In the early decades of baseball, it was not uncommon for a starting pitcher to pitch three hundred innings or more, over the course of a season. In addition, there are accounts of starting pitchers pitching on consecutive days, or even in both games of a doubleheader. It is believed that these feats were only possible because pitchers in the early years of the game, unlike modern starters, rarely threw the ball with maximum effort.
A starting pitcher must complete five innings of work in order to qualify for a "win" in a game he starts. A starter who works six or more innings while giving up three or fewer runs is said to have achieved a "quality start". A starter that finishes the game without having to be relieved by the bullpen is said to have thrown a "complete game". The pitcher that throws a complete game is almost always in a position for a win.
A starting pitcher in professional baseball usually rests three or four days after pitching a game, before pitching another. Therefore, most professional baseball teams have four or five starting pitchers on their rosters. These pitchers, and the sequence in which they pitch, is known as the rotation. In modern baseball, a five-man rotation is most common.