Nippon Professional Baseball consists of two leagues, the Central League and the Pacific League, which both have six teams in each league. There are also two secondary-level professional minor leagues, the Eastern League and the Western League, that play shorter schedules for developing players.
The season starts in late March or early April, and ends in October, with two or three all-star games in July. In recent decades prior to 2007, the two leagues each scheduled between 130 and 140 regular season games, with the 146 games played by the Central League in 2005 and 2006 being the only exception. Both leagues have since adopted 146-game seasons, 73 each at home & on road. In general, Japanese teams play six games a week, with every Monday off.
Following the conclusion of each regular season the best teams from each league go on to play in the "Nippon Series" or Japan Series championship play-off tournament along the lines of the American World Series since 1903.
In 2004, the Pacific League played five fewer games than the Central League teams during the regular season and used a new playoff format to determine its champion (and which team would advance to the Japan Series). The teams in third and second place played in a best-of-three series (all at the second place team's home ground) with the winner of that series going on to play the first place team in a best-of-five format at its home ground. In 2006, the Central League adopted the Pacific League's tournament as well, and the tournament became known as the Climax Series with the two winners, one from each league, competing in the Japan Series.
Comparison with Major League Baseball
The NPB rules are essentially those of the American Major League Baseball (MLB), but technical elements are slightly different: The Nippon league uses a smaller baseball, strike zone, and playing field. The Japanese baseball is wound more tightly than an American baseball. The strike zone is narrower "inside" than away from the batter. Five Nippon league teams have fields whose small dimensions would violate the American Official Baseball Rules. The note set out at the end of Rule 1.04 specifies minimum dimensions for American ballparks built or renovated after 1958: 325 feet (99 m) down each foul line and 400 feet (120 m) to center field.
American Major League Baseball (MLB) players, scouts, and sabermetricians describe play in the NPB as "AAAA"; less competitive than in MLB, but more competitive than in Triple A's (AAA) developing level minor league baseball. Play in the Pacific League is similar to that in American League baseball, with the use of designated hitters, unlike the Central League, which has no DH rule and is closer to National League baseball.
Unlike North American baseball, Japanese baseball games may end in a tie. If the score is tied after nine innings of play, up to three additional innings will be played; this includes the playoffs, but not the Japan Series going beyond Game 7. If there is no winner after 12 innings, the game is declared a tie; these games count as neither a win nor a loss to team standings or to postseason series.
Similar to the current structure of the World Series, a team must win four games to clinch the Japan Series title; however, due to the fact that games can end in a tie, it may take more than 7 games to win the series. If the series must be extended, all games beyond game 7 are played with no innings limit, with game 8 being played in the same venue as game 7, and game 9 and beyond played in the opposing team's venue following a moving day.
Following the 2011 Tōhoku earthquake and tsunami and the ensuing Fukushima Daiichi nuclear disaster, special rules were implemented for the 2011 NPB season:
- For power conservation reasons, besides the usual 12-inning limit, no extra innings were allowed to commence during the regular season once 3 hours, 30 minutes had elapsed from the game start time. This time included delays due to weather, but power failures would result in a called game.
- Due to the delayed start of the season and because of post-season commitments to the champion (such as the 2011 Asia Series), the Japan Series' extension rules were modified in 2011 if the series was tied after seven games, only one extra game would be played.
Most Japanese teams have a six-man starting rotation (as opposed to MLB teams, which feature five-man rotations). Although each team roster has 28 players, similar to other professional sports, there is a 25 player limit for each game. Managers scratch three players before each game, typically including the most recent starting pitchers, similar to professional basketball (two scratches).