The Designated Hitter rule:
A hitter may be designated to bat for the starting pitcher and all subsequent pitchers in any game without otherwise affecting the status of the pitcher(s) in the game. A Designated Hitter for the pitcher must be selected prior to the game and must be included in the lineup cards presented to the Umpire-in-Chief.
The Designated Hitter named in the starting lineup must come to bat at least one time, unless the opposing club changes pitchers. It is not mandatory that a club designate a hitter for the pitcher, but failure to do so prior to the game precludes the use of a Designated Hitter for that game.
Pinch hitters for a Designated Hitter may be used. Any substitute hitter for a Designated Hitter himself becomes a Designated Hitter. A replaced Designated Hitter shall not re-enter the game in any capacity. The Designated Hitter may be used defensively, continuing to bat in the same position in the batting order, but the pitcher must then bat in the place of the substituted defensive player, unless more than one substitution is made, and the manager then must designate their spots in the batting order.
A runner may be substituted for the Designated Hitter and the runner assumes the role of the Designated Hitter.
A Designated Hitter is "locked" into the batting order. No multiple substitutions may be made that will alter the batting rotation of the Designated Hitter.
Once the game pitcher is switched from the mound to a defensive position this move shall terminate the DH role for the remainder of the game. Once a pinch-hitter bats for any player in the batting order and then enters the game to pitch, this move shall terminate the Designated Hitter role for the remainder of the game.
Once a Designated Hitter assumes a defensive position this move shall terminate the Designated Hitter role for the remainder of the game.