Interleague play (NPB)

Interleague play in Nippon Professional Baseball (NPB), often referred to as koryu-sen (交流戦, meaning "interleague games"), is a period of regular season games where teams from the Central League and Pacific League compete against each other. Prior to its introduction, the two leagues operated largely independently during the regular season, with contact limited primarily to the Japan Series.

The koryu-sen was introduced in 2005, driven by a desire to increase competitiveness, fan interest, and overall excitement in NPB. One of the primary aims was to address perceived imbalances between the two leagues, particularly regarding the designated hitter (DH) rule, which is used in the Pacific League but not in the Central League.

The format has evolved over time. Initially, each team played a series of games against every team in the opposite league. Currently, the schedule is typically structured so that each team plays a set number of games against each team in the other league, often with a focus on three-game series. The precise number of games and series structure may vary from year to year.

The outcomes of interleague play games count towards each team's overall regular season record and contribute to determining playoff qualification. The koryu-sen standings are also tracked independently, and a winning league is sometimes symbolically recognized, although this does not directly impact playoff eligibility.

Interleague play has generally been well-received by fans and is now an established part of the NPB season. It provides opportunities for fans to see teams and players they wouldn't normally encounter, fostering broader league-wide interest and offering teams a chance to assess their strengths and weaknesses against different styles of play. The absence of the DH in Central League parks during Interleague play games involving Pacific League teams adds a strategic element to those contests, forcing Pacific League managers to adjust their lineups.