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The Waseda–Keio rivalry (早慶戦 Sōkeisen), also known as the Waseda–KO rivalry, is a college rivalry between two universities located in Tokyo, Japan: Waseda University and Keio University. Both schools are regarded as the most prestigious private universities in Japan. The rivalry dates back to the introduction of baseball in Japan during the Meiji period (1868–1912). In Keio University, it is called Keisōsen, too.
- "[ステンドグラス] 慶早戦の歴史" (in Japanese). Tokyo, Japan: Keio University. 1996. Retrieved 2013-11-19.