Page "Designated hitter" Paragraph 1

In Major League Baseball, the designated hitter is a hitter who does not play a position, but instead fills in the batting order for the pitcher.

DH at the MLB level may be used for the pitcher only as stated in Rule 6. 10.

In any case, use of the DH is optional, however, the manager must designate a DH prior to the start of the game ; failure to do so forfeits the right to use the DH, and the pitcher must then take his turn at bat.

The designated hitter may not play a field position and he may only be replaced by another player not currently in the lineup.

However, the designated hitter may become a position player at any point during the game ; if he does so, his team forfeits the role of the designated hitter, and the pitcher or another player ( possible only in case of a multiple substitution ) must bat in the newly opened spot in the batting order.

The designated hitter could also become the pitcher, in which case any subsequent pitcher, or a pinch hitter, must hit when that spot in the batting order comes up again ( save for a further double switch, as above ).

Likewise, if a pinch-hitter bats for some other player ( such as, hypothetically, the first baseman ) and then remains in the game as the pitcher, the team would forfeit the use of the DH for the remainder of the game, and the player who was serving as the designated hitter would have to assume a field position ( in this hypothetical, play first base save, of course, for switching defensive positions with a teammate.