Major League Baseball tie-breaking procedures

Because inclusion in the Major League Baseball postseason is based upon the teams' regular-season records, procedures exist to break ties between teams.

One-game tiebreakers are played between teams tied for a division championship or a league's second wildcard berth. These games are to be played the day after the season is scheduled to end. Home-field advantage is determined using the rules listed below ("Breaking Ties Without Playoff Games").

From the implementation of the wildcard in 1994 to the end of the 2011 season, a different rule was in place. Two teams tied for a division did not play a tiebreaker if their records were better than all non-division winners in their league. Instead, such a tie was broken using the rules listed below ("Breaking Ties Without Playoff Games"). This scenario happened in the 2001 Major League Baseball season when the Houston Astros and St. Louis Cardinals tied for first place in the National League Central. The Astros were awarded the division title by virtue of a better head-to-head record against the Cardinals, while St. Louis received the wildcard berth.

With, however, the adoption of a second wildcard berth and a wildcard game beginning in the 2012 season, the non-division winner with the best record in the league faces possible elimination on the first day of the postseason. As a result, the tie-breaking rules were changed so that two teams tied for a division championship must play a tie- breaking game even if both teams have already qualified for the postseason. The team losing the tie-breaking game will now qualify for a wildcard berth only if its regular-season record is among the league's two best records for non-division-winners. If that team is tied for the second wild-card spot, a second tie-breaking game would then be played.

If, on the other hand, two teams are tied for the first wildcard slot, no tie-breaking game is played. Rather, the two teams simply play against each other in the wildcard game, with home-field advantage awarded using tie-breaking rules described in the next section.

If two champions from separate divisions have the same record, the tiebreaking procedure listed above is used to determine postseason seeding. No additional games are played.

...
Wikipedia