In baseball, the double switch is a type of player substitution, usually performed by a team while playing defense. The double switch is typically used to make a pitching substitution, while simultaneously placing the incoming pitcher in a more favorable spot in the batting order than was occupied by the outgoing pitcher. (On the assumption that the pitcher will be a poor hitter, the incoming pitcher will generally take the spot in the batting order of a position player who has recently batted, so as to avoid the pitcher making a plate appearance in the next couple of innings.) To perform a double switch (or any other substitution), the ball must be dead.
Since the batting order can be changed only as a result of a player substitution,[Note 1] while the defensive arrangement may be changed freely (among players currently in the game), the double switch typically takes the following form:
In the short term, the lineup is strengthened because a poor-hitting pitcher will not make a plate appearance soon. The disadvantage is that a position player must be removed from play and replaced by another, often inferior, position player. The advantage of the double switch over pinch hitting is that it uses up fewer players. If a relief pitcher is brought in before the at-bat, then the manager can substitute a pinch-hitter for him. However, this would require a new pitcher for the next half-inning. By using a double switch, an incoming pitcher can be left in the game for a substantial period before his turn in the batting lineup arrives, no matter what the previous batting order was.
While the double switch plays an important role in the National League, the designated hitter (DH) rule has effectively eliminated the advantages of the double switch in the games using American League rules--so the double switch is rarely used in AL games. The designated hitter's role is to bat in the pitcher's spot in the lineup. Major League rules do not allow a multiple substitution involving a DH to alter the lineup position of the DH. Although uncommon, it is possible to forgo the DH privilege (e.g. if the DH becomes a position player, usually due to injury to another player), and then utilize the double switch later with that player. It can also be used by an AL team playing on the road during interleague play or the World Series, because MLB rules call for the rules of the home team to be used when teams from different leagues meet (so there is no DH when the NL team is the home team).
When the team is up to bat, a manager can get the same effect as a double switch by leaving in the player who has pinch-hit for the pitcher and replacing another player in the lineup who has made the last out of the inning with a new pitcher. This will take the following form:
A double switch has infrequently resulted in a team batting out of turn because the lineup card was not updated to reflect the change, either because the umpires were not informed of the change, or because the change was not recorded. In addition, because double-switches are typically communicated verbally, it creates opportunities for confusion and miscommunication that can be costly to the switching team. 
This code of rules is written to govern the playing of baseball games by professional teams of the American League of Professional Baseball Clubs, the National League of Professional Baseball Clubs, and the leagues which are members of the National Association of Professional Baseball Leagues.(Current edition available online from Major League Baseball.)