The Climax Series is the Nippon Pro Baseball equivalent of the League Championship Series in Major League Baseball - a postseason series used to determine the team who will represent the league in the Nippon Series.
Like it had been the case in MLB before 1969, only two teams could advance to the Nippon Series originally. The Pacific League tried to form a new playoff system in order to increase the league's popularity in 1974. The team that had best first-half record would be matched against the team with the best second-half record, and they would hold a best-of-five series to determine the team which would represent the PL in the Nippon Series. This system proved problematic as sometimes a team would win both halves of a season, and it was eliminated in 1982.
The Pacific League didn't give up, and they re-created the playoff system in 2004. The second-place team and the third-place team would hold a best-of-three series, then the winner and the first-place team played each other in a best-of-five series. Originally, the PL Championship winner only received home field advantage, but the league soon decided to automatically award the first-place team a win because the PL Champion Daiei Hawks were beaten by the second-place team for the second consecutive year in 2006. Ironically, the new system helped the PL Champion Hokkaido Nippon-Ham Fighters beat the Hawks in the 2007 PLCS.
On the other hand, the more conservative Central League didn't attempt to implement a playoff system until they lost the Nippon Series for the three consecutive years after the PL created their playoff. Thus, the CL decided to copy the PL's system, and the two leagues renamed it the Climax Series. The CL wanted to give the CL Champion winner an automatic win, but the Yomiuri Giants opposed this, so they just gave the winner home field advantage. However, after the Giants were swept by the Chunichi Dragons in 2007 CLCS, they reinstated the automatic win. The final stage of the Climax Series was a best-of-five series, and it was changed to a best-of-seven series in 2008.