Before a sanctioned baseball game begins, both teams must supply the head umpire with a written batting order. This list of eligible players is also shared with the media covering the game, the game announcers and the players themselves. Under rule 6.01(a) of the official baseball rules, this batting order must be strictly observed unless team managers notify the head umpire of legal substitutions. Violating this rule is known as batting out of turn or batting out of order.
The act of batting out of turn can be the result of a miscommunication between a player and the team manager who established the batting order. The head umpire's list is considered to be the official order, but sometimes the unofficial list in the players' dugout is accidentally transposed. Fortunately, under the rules any player batting out of order can be recalled by the batting team if a mistake is discovered. The proper batter simply takes his place and assumes the strike and ball count.
A player may also assume he will be batting in the same order as previous games and inadvertently step up to the plate out of order. Again, if the error is discovered in time by the batting team, the player can be replaced with the proper batter in the order. If the improper batter does get a base hit or home run, however, then a complicated set of penalties can be applied.
It's these potential penalties which can make the "batting out of turn" rule very confusing to fans and players alike. Ordinarily, a batter who bats out of order, but is not noticed by either team is considered a legitimate batter and play continues, as long as the defensive team pitches a ball to the next batter or makes any other defensive play, i.e. a pick-off. The defensive team must object to the improper out-of-order batter before the next play starts or else the batter is allowed to remain on the base or is credited with a home run.
If the defense does ask the umpire for a ruling, a number of things can happen. If player A is improperly followed by player C and the defense objects, player C can be removed from the bases and any scoring runs due to his efforts would be nullified. The proper batter who should have followed player A, player B, could be declared out before ever stepping up to the plate. In an ironic twist, the next proper batter in the order would be player C, the same batter who caused the problem by batting out of turn in the first place. Player C would actually be at-bat twice, although the team would be penalized an out.
There is a significant amount of strategy built into the batting order, with stronger hitters going first and weaker hitters batting last. The first batter is generally a strong base runner with a good batting average but not generally a power hitter. The best power hitter generally bats third, with a clean-up power hitter in the fourth spot. Strategically, batting out of order may be to a batting team's advantage if bases are loaded and a power hitter may be able to clear the bases with a home run. Considering the penalties of batting out of turn, however, the benefits may not outweigh the risks.